I have a dropdownlist in my web page and two command buttons and a tabcontainer. The tabcontainer does not appear after the load of page according to my code.

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load          If Not IsPostBack Then                    TabContainer1.Visible = False                   If CtvAct.GetRecords("Fill_RequestTypeTb") = True Then                          ReqTypeCmbo.DataSource = CtvAct.MainDataset.Tables("tbOLRequestType").DefaultView                          ReqTypeCmbo.DataTextField = "RequestTypeName"                          ReqTypeCmbo.DataValueField = "RequestTypeId"                          ReqTypeCmbo.DataBind()                          Dim itm As New ListItem                          itm.Text = "-- ??? ??? ????? --"                          itm.Value = "-1"                          itm.Selected = True                          ReqTypeCmbo.Items.Insert(0, itm)                          ReqTypeCmbo.SelectedIndex = 0                  End If          End If  End Sub    Protected Sub PrntCmd_Click(ByVal sender As Object, ByVal e As EventArgs) Handles PrntCmd.Click          TextBox6.Text = "gggg"  End Sub 
Protected Sub ReqTypeCmbo_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs) Handles ReqTypeCmbo.SelectedIndexChanged         If ReqTypeCmbo.SelectedItem.Text = "????" Then             TabContainer1.Visible = True             TabContainer1.ActiveTabIndex = 0             OwnerDataPnl.Enabled = True             AttornyDataPnl.Enabled = True             BaseDataPnl.Enabled = True             RenwDataPnl.Visible = False             NewOwnerDataPnl0.Visible = False             AddActivtyPnl.Visible = False             SubstitPnl.Visible = False             FinishLicePnl.Visible = False             CancelActivityPnl.Visible = False             LocTransPnl.Visible = False             ' Panel1.Visible = False             '?????? ?????? ??????? ????????             REstOfficeAddrTxt.Enabled = True             REstOffIdeIssuedPlaceTxt.Enabled = True             REstOffIdentityNameTxt.Enabled = True             REstOffIdentityNumberTxt.Enabled = True             REstOffIdentityStartDateTxt.Enabled = True             REstOffMobileTxt.Enabled = True             REstOffNameTxt.Enabled = True             REstOffPhoneTxt.Enabled = True             OwnerShipNumberTxt.Enabled = True             OwnerShipDateTxt.Enabled = True             OwnerShipIssuedPlaceTxt.Enabled = True             SerailTxt.Enabled = True             RequestIdTxt.Enabled = True             RequestDateTxt.Enabled = True             RenwDataPnl.Visible = True              ''''             If CtvAct.GetRecords("Fill_NationalityTb") = True Then                 OwnrNationCmbo.DataSource = CtvAct.MainDataset.Tables("tbOLNationality").DefaultView                 OwnrNationCmbo.DataTextField = "NationName"                 OwnrNationCmbo.DataValueField = "NationId"                 OwnrNationCmbo.DataBind()                 Dim itm As New ListItem                 itm.Text = "-- ??? ??????? --"                 itm.Value = "-1"                 itm.Selected = True                 OwnrNationCmbo.Items.Insert(0, itm)                 OwnrNationCmbo.SelectedIndex = 0               End If             If CtvAct.GetRecords("Fill_ActivityTb") = True Then                 AcivityCombo.DataSource = CtvAct.MainDataset.Tables("tbOLActivity").DefaultView                 AcivityCombo.DataTextField = "ActivityName"                 AcivityCombo.DataValueField = "ActivityId"                 AcivityCombo.DataBind()                 AcivityCombo.SelectedIndex = -1                 Dim itm As New ListItem                 itm.Text = "-- ??? ?????? --"                 itm.Value = "-1"                 itm.Selected = True                 AcivityCombo.Items.Insert(0, itm)                 AcivityCombo.SelectedIndex = 0               End If               'If CtvAct.GetRecords("Fill_ActivityTb") = True Then             '    DropDownList2.DataSource = CtvAct.MainDataset.Tables("tbOLActivity").DefaultView             '    DropDownList2.DataTextField = "ActivityName"             '    DropDownList2.DataValueField = "ActivityId"             '    DropDownList2.DataBind()             'AcivityCombo.Visible = True             ' LbCon.Text = CtvAct.Prt              'End If              AttronayNationCmbo.SelectedIndex = -1             If CtvAct.GetRecords("Fill_NationalityTb") = True Then                 AttronayNationCmbo.DataSource = CtvAct.MainDataset.Tables("tbOLNationality").DefaultView                 AttronayNationCmbo.DataTextField = "NationName"                 AttronayNationCmbo.DataValueField = "NationId"                 AttronayNationCmbo.DataBind()                 Dim itm As New ListItem                 itm.Text = "-- ??? ??????? --"                 itm.Value = "-1"                 itm.Selected = True                 AttronayNationCmbo.Items.Insert(0, itm)                 AttronayNationCmbo.SelectedIndex = 0                 ' LbCon.Text = CtvAct.Prt             End If             If CtvAct.GetRecords("Fill_LocationTb") = True Then                 LocationIdCmbo.DataSource = CtvAct.MainDataset.Tables("tbOLLocation").DefaultView                 LocationIdCmbo.DataTextField = "LocationName"                 LocationIdCmbo.DataValueField = "LocationId"                 LocationIdCmbo.DataBind()                 Dim itm As New ListItem                 itm.Text = "-- ??? ?????? --"                 itm.Value = "-1"                 itm.Selected = True                 LocationIdCmbo.Items.Insert(0, itm)                 LocationIdCmbo.SelectedIndex = 0                 ' LbCon.Text = CtvAct.Prt             End If             ''''''''''''''''''''''''''''''''             con = New SqlConnection(CtvAct.Connection_String())             CmdActivty = con.CreateCommand             CmdActivty.CommandText = "SELECT ActivityId FROM tbOLStoreActivty"             DaActivity.SelectCommand = CmdActivty             DaActivity.Fill(DsActivity, "tbOLStoreActivty")             ActivityGV.DataSource = DaActivity             ActivityGV.DataMember = "tbOLStoreActivty"              AcivityCombo.DataSource = ds.Tables(0)             AcivityCombo.DataTextField = "ename"             AcivityCombo.DataValueField = "eid"             AcivityCombo.DataBind()              '''''''''''''''''''''''''''''             LbCon.Text = CtvAct.Prt         ElseIf ReqTypeCmbo.SelectedItem.Text = "?????" Then             TabContainer1.Visible = True             TabContainer1.ActiveTabIndex = 1             OwnerDataPnl.Enabled = False             AttornyDataPnl.Enabled = False             BaseDataPnl.Enabled = True             AddActivtyPnl.Visible = False             SubstitPnl.Visible = False             FinishLicePnl.Visible = False             RenwDataPnl.Visible = True             AddActivtyPnl.Visible = False             NewOwnerDataPnl0.Visible = False             CancelActivityPnl.Visible = False             LocTransPnl.Visible = False              '?????? ?????? ??????? ????????             REstOfficeAddrTxt.Enabled = False             REstOffIdeIssuedPlaceTxt.Enabled = False             REstOffIdentityNameTxt.Enabled = False             REstOffIdentityNumberTxt.Enabled = False             REstOffIdentityStartDateTxt.Enabled = False             REstOffMobileTxt.Enabled = False             REstOffNameTxt.Enabled = False             REstOffPhoneTxt.Enabled = False             OwnerShipNumberTxt.Enabled = False             OwnerShipDateTxt.Enabled = False             OwnerShipIssuedPlaceTxt.Enabled = False             SerailTxt.Enabled = True             RequestIdTxt.Enabled = True             RequestDateTxt.Enabled = True             '''''''''          ElseIf ReqTypeCmbo.SelectedItem.Text = "??? ?????" Then             TabContainer1.Visible = True             TabContainer1.ActiveTabIndex = 1             OwnerDataPnl.Enabled = False             AttornyDataPnl.Enabled = False             BaseDataPnl.Enabled = False             RenwDataPnl.Visible = False             NewOwnerDataPnl0.Visible = True             AddActivtyPnl.Visible = False             SubstitPnl.Visible = False             FinishLicePnl.Visible = False             CancelActivityPnl.Visible = False             LocTransPnl.Visible = False         ElseIf ReqTypeCmbo.SelectedItem.Text = "????? ????" Then             TabContainer1.Visible = True             TabContainer1.ActiveTabIndex = 1             OwnerDataPnl.Enabled = False             AttornyDataPnl.Enabled = False             BaseDataPnl.Enabled = False             RenwDataPnl.Visible = False             NewOwnerDataPnl0.Visible = False             AddActivtyPnl.Visible = False             SubstitPnl.Visible = False             FinishLicePnl.Visible = False             CancelActivityPnl.Visible = True             LocTransPnl.Visible = False         ElseIf ReqTypeCmbo.SelectedItem.Text = "????? ????" Then             TabContainer1.Visible = True             TabContainer1.ActiveTabIndex = 1             OwnerDataPnl.Enabled = False             AttornyDataPnl.Enabled = False             BaseDataPnl.Enabled = False             RenwDataPnl.Visible = False             NewOwnerDataPnl0.Visible = False             AddActivtyPnl.Visible = False             SubstitPnl.Visible = False             FinishLicePnl.Visible = True             LocTransPnl.Visible = False         ElseIf ReqTypeCmbo.SelectedItem.Text = "??? ???? / ????" Then             TabContainer1.Visible = True             TabContainer1.ActiveTabIndex = 1             OwnerDataPnl.Enabled = False             AttornyDataPnl.Enabled = False             BaseDataPnl.Enabled = False             RenwDataPnl.Visible = False             NewOwnerDataPnl0.Visible = False             AddActivtyPnl.Visible = False             SubstitPnl.Visible = True             FinishLicePnl.Visible = False             LocTransPnl.Visible = False         ElseIf ReqTypeCmbo.SelectedItem.Text = "??? ????" Then             TabContainer1.Visible = True             TabContainer1.ActiveTabIndex = 0             OwnerDataPnl.Enabled = True             AttornyDataPnl.Enabled = True             BaseDataPnl.Enabled = True             RenwDataPnl.Visible = False             NewOwnerDataPnl0.Visible = False             AddActivtyPnl.Visible = False             SubstitPnl.Visible = False             FinishLicePnl.Visible = False             CancelActivityPnl.Visible = False             LocTransPnl.Visible = True         End If     End Sub

If I press any button after page load the button work very selecting any item from dropdownlist make the tabcontainer appear but after the tabcontainer appear, the buttons does not work (postback) how can I solve these problems