Dell Latitude D520 laptop flat screen not working - works fine on boot up

Hi I have a Dell Latitude D520 laptop with a flat screen.  I have a second flat screen monitor acer hooked up.  Everything was working fine since I put the second screen on over a month ago.

Today I was working fine.  I closed the laptop to take a break. When I came back the second monitor, which is now the main monitor since I hooked it up - works fine.  The laptop works fine.  I went to drag a file to the laptop monitor the number 2 monitor and it would not come over.  I looked at the screen on the laptop and it was blank.

I went to the troubleshooting in the display section and it did not help at all.  I did notice however, that when I restarted the computer the laptop screen works fine.  After the start up though it is blank again.  All the lights are normal on restart.  I'm wondering if I hit a key and turned it off somehow when I shut the lid for my break

: - )  not sure.

I am using windows XP.

Thank you for any help at all!

Bluedolphin Crow