Well you will have to open it up, the laptop, and check the ribbon cable to start. Make sure there are no dents, bends, cuts, tears, etc., in it and you should also make sure it isn't loose which would cause the display not to work.
If the cable seems good, you have the option to try and get a new cable, just to be sure.
If none of that fixes it, then it is the display that is going to need replacing.