Hi tourangelle, it's a software engine problem. You have just to software update your phone.
Here, this the software that you have to use:
http://www-support-downloads.sonymobile.com/Software%20Downloads/Update_Service_Setup-2.11.12.5.exe
Before anything, your battery must be fully charged.
Install it on your PC.
Then run the program.
Choose the model.
Launch.
Take out the battery, put it back, and keep the back button pressed.
Plug it to your computer (don't forget, you have to keep the back button pressed).
When the software update starts, you can release the back button.
Wait until it finish, restart your phone.
It's finish..