I did something stupid. When my Dell Vostro 1000 laptop screen cracked and went all crazy and I couldn't see anything, I attached it to an external monitor and pressed Fn + F5 and it worked.

But then I did something in the control panel that made the external screen look similar to the laptop screen. That made the external screen completely unusable too. In the control panel it was the last tab – something with "dual screens" I think.

Is there anyway to reverse what did, turn off the laptop screen permanently and turn the external monitor into the primary?

Remember that I can't see anything on the laptop screen. It is a complete goner – and because of what I did I can't see anything on the external monitor.