if it's a laptop it could be as above, but I'd say it's more likely to be either.
The little switch that shuts the screen off when shut has got stuck. look for a little pin near the base of the screen on the main unit that may be stuck in and see if you can flick it back up with a pin or similar.
Or there is a problem with either the back light tube having blown or the voltage inverter that supplies the power to the tube has blown, neither if these are particularly expensive jobs to sort out, you'd probably be looking at about �70 ish by my reckoning if that's the problem.