Assuming you're not rooted, try wiping the cache partition and resetting from the recovery menu.
Recovery menu is a feature built into Android smartphones that allows certain functions to be carried out in the event your phone isn't performing correctly. It can be accessed by a combination of buttons and is navigated using the volume and power buttons.
To wipe the cache partition, follow these steps,
1. Turn your device off.
2. Press and hold the volume up button, the home button, and the power button at the same time.
3. When the phone vibrates, let go of the Power button, but continue to told the volume up button and the home button.
4. Once you get to the Android Recovery Screen, let the buttons go.
5. Use the volume down button to highlight 'wipe cache partition.'
6. Press the power key to confirm the selection.
7. Reboot System Now should be selected, then press the Power Button to confirm.
Attempt to start the handset, if the issue continues, repeat steps 1-4.
5. Select 'wipe data / factory reset'.
6. Press Power to comfirm the selection.
7. Select delete all user data
8. Press the power button to confirm.
9. Once that completes press power to confirm the Reboot System Now selection
If that fails, you likely have hardware failure or corrupted software, which means your phone is defective. If you are within your 1 year manufacturer warranty period, your phone should be serviced/replaced.
Since you have performed the factory reset, the issue still remains, and you're not rooted, it's a sign of a defective device. There is no possibility that it is a service provider issue, unless they recently released an OTA update that was corrupted. However, you said that the issue began when you attempted to reset the phone, which makes the latter highly unlikely. (As well as the fact that the update is provided to the service provider by Samsung.)
If you're within your warranty period (1 year) or you have handset insurance on your line, you can visit an AT&T store to get help requesting warranty fulfilment or initiate an insurance claim.
If you're out of warranty, you do not have an insurance policy, and have a PC running Microsoft Windows or Mac OSX (Linux is not supported) with the Kies Software installed (you can download it from here), you can try these steps Samsung provides to reload the Android OS onto your phone. (If your phone is supported and the software is available.)