Chosen Solution
I have buy used orignial battery for iPhone 7. At first i t was fine but after one month is suddenly shuts down and will not turn on, or starts to turn on but gets stuck during startup. Now the phone is dead. Is it possible that battery is dead? how to check it?
Plug the phone into computer, open up iTunes and see if it detects there. If it does detect there then the issue is related to the board and not the battery. More info about iPhone 7 bootloop issues here: Stuck on Apple logo Look for jessabethany’s post.
