Hi there and welcome
From what you described this is the internal memory that's faulting
The firmware runs from this and it cannot be read correctly, this is why it is not starting correctly.
Unfortunately this issue requires the mainboard to be replaced, which is a costly repair if you need to pay for it yourself.
But your only option Carolyn is to have the mainboard replaced under warranty or consider having the phone replaced as replacing the mainboard at your own expense is not worth it in the long run.
It is covered under the warranty Carolyn, insurance is when you caused the issue, ie: Dropping it in water, which you did not, AT&T need to replace it under warranty for you at no cost.
If you would like any additional information or assistance, please do not hesitate to let me know.
Else if you can take a second to rate my service by clicking one of the stars at the top of the screen then submit, that would be greatly appreciated.