Black screen in dos mode, ok in Windows.

I own a Dell Isnpiron 1720 laptop. I went a few days ago to a friend to clean the interiour of my laptop. My friend cleaned it up, but afterwards my display didn't worked. It has a very strange problem. It I turn the laptop on I can not see any boot text, no image, no nothing. It used to say Dell Inspiron 1702, and the usual hit F2 for bios, etc. My screen remains black untill a point when Windows 7 starts. From there I can use the laptop display. When I open Nvidia Control Panel, above the monitor it sais dissconected.... I also have no Flat Panel Scalling... If I try to connect the laptop through a vga cable with an externel monitor it works perfectlly. But, when I remove the vga the laptop's display stays black... If I start the laptop with an external monitor and I remove it later, the laptop display stays black... If I try to get to safe mode, It won't work without an external display. It loads, but I can not see anything on my laptop display... Can anyone tell me what's wrong? I cleaned the wires and the slot from the monitor to the video card, I went to a computer servie and they couldn't help me..