I have had it too. It must be the screens actually I had two out of four. I tried everything I could including putting the screen on a phone that worked. No fix for front camera but I can tell you how to unlock it.
Go to setting/general/reset (scroll to bottom)/reset all settings.
It will ask for confirmation.
After I did this the back camera will worked, but if I try the front it locks up, this affects facetime as well.
Note this only changes settings like background, ringtones, etc. does not effect data and pretty quick to do.