If the Display is black, but the Desktop can still be seen a bit, this means, that the background light has died.
Some of the errors could be:
- A defective inverter or CCFL tube
- A broken cable
- The switch detecting whether the lid is open or closed could be stuck
- There could be a damage on the Mainboard.
In most cases it is probably best if you send your device in for repair to a specialist.