Heres what you do:
In a bright room, power up the laptop without the LCD projector. Look very closely at the screen. Do you see a faint image of the desktop, startup .... ? If so, the back lighting has gone out. If nothing - continue:
If you see nothing at all: try pressing your FN + LCD keys (like F7 or F8 i forget). If pressing that doesn't help AND YOU DO NOT EVER SEE THE DELL LOGO, you may have a loose screen cable or bad screen.