I faced the same problem once.
Steps to fix.
1. Open the back lid and Press and hold the power button for 10 seconds (You may need a pen or something). 3 vibrations and phone will be off.
2. Uninstall PC companion and SEUS and clear everything in C:\ProgramData\Sony Mobile\Update Engine\db\13740270\blob_fs (Update files)
3. Install SEUS (DONT PLUG IN YOUR PHONE or again It will be in bootloop)
4. Select the phone model in SEUS and press next.
5. When it prompts , press and hold down volume key and plug it in
HOPEFULLY this will fix the issue.
Windows 8 may give some issues. I used windows 7 after I git the error.