I had a friend 2 weeks ago who had his run over by 3 cars, busted the screen, replaced it, and powered up, but after testing eveything, the phone would shutdown when using the camera, found the rear camera was slightly bent which broke it.
Basically, look very carefully at everything inside! See if anything looks slightly bent! Compare pictures of the motherboard to see if any chips have come off, and just try disconnecting the front facing camera, usually this will power up but with a nand flash error if there is a problem with this componant, but it can't hurt trying.