Repaired iPhone, now phone is not turning on

Hi

My iPhone's back camera is quite temperamental and sometimes if dropped disconnects and doesn't work - I know the problem is not that the camera is broken only that it is a connection problem because I have fixed the issue before.

This time, after putting my phone back together the phone will not turn back on at all. Itunes recognises the device (however I can't restore it because I can't respond on my iPhone when prompted to allow the computer access). I have tried to hard reset using the home and power buttons and it did not work. The only problem I had when repairing the phone was losing a screw for the logic board cover but I didn't think this would cause this kind of issue?

Thanks