Can you try to perform a software/firmwware repair using PC Companion to see if ot helps.
Connect your phone to the PC via the USB cable
The repair option on PC Companion (Support Zone --> Update --> Repair).
Please backup your phone data and settings before proceeding.
If it is NOT TURNING ON,.. PLEASE try:
1.Go to PC companion -> Sony Update Service and select phone update without connecting your phone.
2. Select your device and download the update (DONOT CONNECT PHONE)
3. When the download completes it will prompt for connecting you phone.
4. Before that phone must be switched OFF (donot turn on).
JUST PRESS DOWN THE VOLUME DOWN KEY AND CONNECT THE USB CABLE.
5. The Phone's LED will glow green and the update for phone will start.
6. After the update completes you will get the phone as an fresh new Operating System.
(WARNING - YOU WILL LOOSE ALL YOUR DATAS AND GET A FRESH START)