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Was there also a message about software not authorized being found on your phone?
If so, try a hard reset (strictly a Wipe Data & Factory Reset, done by holding down several buttons then pressing POWER when switching On, in extreme summary, and capable of wiping all individual data off the phone). You will lose all data on the phone and your will have to reconfigure it from scratch (since it will be back to factory defaults), but it will work again after that.
For details, if that is something you want to try, just Reply To Expert and let me know, please. Thanks.
Then that is what you should do: take it to an ATT store. The choices are either that, or try a Wipe Data & Factory reset.