It sounds like the back-light in your laptop's LCD screen has died. If you turn the laptop on, and hold the screen up to the light, you may be able to see the operating system and windows, but since the screen isn't emitting any light, it appears to be off or all black. If this is the case, you'll need to either replace the back-light, have the laptop repaired, or replace the whole thing. There's no easy fix for a dead back-light.