The screen is lit by LEDs, which receive power via a small ribbon cable. This cable is delicate, and is located near the front/bottom of the phone. This may have been dislocated in it's connector, either on impact, or while the repairer dismantled the phone. If you (or a competent friend) could carefully disassemble it, then it would be quite an easy fix. Apart from that, the only other explanation is the OS on the phone, which I think Is allot less likely.
Sent from my HP Slate 7