Make sure the battery is nearly fully charged. Go to your local AT&T store and ask them where the Device Support Center is. (It should be close.) Bring the phone to the DSC and tell them what happened - just that it was updating and froze, not about trying to flash other software. They'll flash it for you.
(If they can't, ask for the phone number to call, because this is a liability issue. Then call them and explain that their update ruined your phone, and the DSC can't fix it, so you'd like AT&T to replace it. They will. With a refurb, but yours is used too.)
05-28-2018 11:07 PM