I've been developing a Sharepoint web part, which had been working fine upon deployment.  The web part has been developed with WSP Builder, packaged up and then deployed via stsadm.  The web part has been deployed tens, if not a hundred times to the dev box with no problems.  Now, the web part throws an error which breaks the page it's on:
  Object reference not set to an
  instance of an object.  Description:
  An unhandled exception occurred during
  the execution of the current web
  request. Please review the stack trace
  for more information about the error
  and where it originated in the code. 
  
  Exception Details:
  System.NullReferenceException: Object
  reference not set to an instance of an
  object.
  
  Source Error: 
  
  An unhandled exception was generated
  during the execution of the current
  web request. Information regarding the
  origin and location of the exception
  can be identified using the exception
  stack trace below.  
  
  Stack Trace: 
  
  [NullReferenceException: Object
  reference not set to an instance of an
  object.]
  NYCIRB.DMS.WebParts.SearchUpload.SearchUpload.HandleException(Exception
  ex) +62
  NYCIRB.DMS.WebParts.SearchUpload.SearchUpload.OnLoad(EventArgs
  e) +214
  System.Web.UI.Control.LoadRecursive()
  +50    System.Web.UI.Control.LoadRecursive()
  +141    System.Web.UI.Control.LoadRecursive()
  +141    System.Web.UI.Control.LoadRecursive()
  +141    System.Web.UI.Control.LoadRecursive()
  +141    System.Web.UI.Control.LoadRecursive()
  +141    System.Web.UI.Page.ProcessRequestMain(Boolean
  includeStagesBeforeAsyncPoint, Boolean
  includeStagesAfterAsyncPoint) +627
When looking through my Sharepoint logs, I find these errors repeated over and over which correspond to the time the web part was attempted to be loaded:
  01/19/2009 10:53:14.43    w3wp.exe
  (0x05E0)
    0x00FC	Windows SharePoint Services
    General
    72kg	High    	(#2: Cannot open
  "Resources.en-US.resx": no such file
  or folder.)     01/19/2009 10:53:14.43
    w3wp.exe (0x05E0)
    0x00FC	Windows SharePoint Services
    General
    8e26	Medium  	Failed to open the
  language resource for
  Fea367b94a9-4a15-42ba-b4a2-32420363e018
  keyfile Resources. 01/19/2009
  10:53:17.55   w3wp.exe (0x05E0)
    0x00FC	Windows SharePoint Services
    General
    8e25	Medium  	Failed to look up
  string with key "XomlUrl", keyfile
  core.   01/19/2009 10:53:17.55
    w3wp.exe (0x05E0)
    0x00FC	Windows SharePoint Services
    General
    8l3c	Medium  	Localized resource for
  token 'XomlUrl' could not be found for
  file with path: "C:\Program
  Files\Common Files\Microsoft
  Shared\Web Server
  Extensions\12\Template\Features\Fields\fieldswss.xml".
  01/19/2009 10:53:17.55    w3wp.exe
  (0x05E0)
    0x00FC	Windows SharePoint Services
    General
    8e25	Medium  	Failed to look up
  string with key "RulesUrl", keyfile
  core.   01/19/2009 10:53:17.55
    w3wp.exe (0x05E0)
    0x00FC	Windows SharePoint Services
    General
    8l3c	Medium  	Localized resource for
  token 'RulesUrl' could not be found
  for file with path: "C:\Program
  Files\Common Files\Microsoft
  Shared\Web Server
  Extensions\12\Template\Features\Fields\fieldswss.xml".
I've retracted the web part manually through Solution Management, retracted through stsadm, checked for the existence of the resource file, which is nowhere to be found.  I'm pretty much at a loss to why this happened or how to resolve it.