Backlight is broken or the switch is not functioning correctly which I suspect is the cause of your problem. I took apart and put my laptop back together with the help of the engineers manual for my specific model, it was really annoying and I had to make notes for each stage. I think newer laptops are LED but my old ones use lamps, if it's a lamp it'll be near the hinge, and the switch is visible on most laptops I've seen. I've seen lamps being faulty (on Samsung TVs) but never witnessed one breaking, it's usually something else that goes first, so I'd check the switch first, see if it's functioning. I have no idea how to repair it.