Dell Studio 1558.. Laptop comes on, screen lights up but is blank. I connected an external monitor and it comes right up. All other peripherals work fine. What can this be?
If there's a faint image on the screen (use a very bright light to check), it's likely the backlight that's bad.
If not, hold the D key through powerup - you should see a series of screen tests. If you don't, the screen is bad.
If you do see that, but don't get any image at all, the GPU is likely bad (they can fail in such a way that the display is OK externally but faulty internally).