I changed the rear housing on an iPad 2, which requires you to remove the battery and upon completion I powered it on and as soon as it made it to the lock screen I slid to unlock and 2-5 seconds after seeing the home screen it would go blank and the apple screen would come up and repeat (loop) and if I did not unlock it, the same thing would happen within 5 seconds. The battery was not in the red.
My fix was to go back in and disconnect the battery and clean the contacts with alcohol, air dry and reassemble. I don't seal the screen in until all systems are go.
This worked! No loop! Good luck...