iPhone 4 backlight is working but LCD display does not

My friend recently had his little brother take his iphone apart as an experiment and ended up removing the 2 black EMI shields and losing the screws that hold the larger EMI shield in place as well as the non grounding screw for the shield for the flex cable (the grounding screw is still there) and the right screw for the bottom speaker part and the two bottom screws that are used to hold the back cover in. Now that I've tried to trouble shoot the problem, it seems that no matter what screen I put in (even working ones) only the backlight comes on and the LCD stays black. I was wondering if this could possibly be a display problem on the logic board or if it was because of the missing parts. Thank you guys!