Hello,
Sorry to hear you are having this issue. Hopefully, you've backed up our phone at this point because you will either need to take your phone in to a local carrier for assistance, or do a Factory Reset using the hardware keys.
To do the Factory Reset:
1) Press and hold the following keys at the same time [while power off]: Power/lock key + Volume down key.
2) When phone vibrates, release keys
3) repeatedly tap the Back key + Home Key until the green Android Robot animation appears.
4) The phone will perform a hard reset. If AT&T logo appears, start over from step 1.
Hope this helps.
^CL