Make sure the LCD cable is plugged in properly, into the board and screen. Most of the time this happens due to the cable not being fully pushed into the logic board, so it is only lighting a number of LEDs for the screen. You can try brushing the inside of the LCD connector with a soft brush and 95%+ isopropyl, in case there is dirt inside, stopping a proper connection.
It can be caused by other things (LCD, LED return resistors, etc) but I would say the above is the most likely reason, if it was fine when you had it. The cable probably wasn't secured properly and came slightly loose.