Why my PowerBook G4's screen won't turn on?

I've got a Powerbook G4 15" model that's screen won't turn on.

You can hear the computer start up normally (apple sound), the dvd drive even starts running and the fans kick in and then stop. Meanwhile on the screen, well there's still nothing! It stays black.

I once managed to start it up holding down a certain key (I can't remember which:( ) I even managed to take out a dvd that stayed in, I turned it off and then it wouldn't turn on again.

All possible solutions are welcomed Thanks