Share your experience!
Yesterday I updated my phone through PC Companion, I followed every step correctly and the last step was to turn on the phone and wait, but mine didnt turn on. It keep the screen with the sony logo for hours and hours until the battery died so I charged it (through the wall) and kept waiting more and still the same result.
Hi @Rocker_Honey,
I'm sorry to hear this. 
You can also try pressing the volume up (+) key and the power key together at the same time for 5-10 seconds and then try to power on the phone again. If it still won't power on properly you will have to try a software repair using PC Companion.
Thanks for the tip about pushing the start button while holding the volume up button for 10 seconds. It worked like a charm. I too had done a software update, although not through a PC. I am very happy with my Sony and was shocked when the phone wouldn't start up. Thanks again from a happy customer!
Ok so my phone died this morning. I can reset via the little "OFF" button. But as soon as I plug a charger in or the USB cable to the PC I get a quick red LED flash once and it turns off. The PC companion doesn't connect to the phone. I've done the holding power and the volume up, does not fix the issue.
HELP PLEASE!
have you exposed the phone to water? have you dropped the phone?
remove the SD and SIM cards > connect the phone to a wall charger > press and hold power & volume up for 10-15 seconds > let it charge for 2-3 hours > then repair it with PCC > DO NOT turn on the phone or attempt to turn it on.
software repair using PC Companion
Bridge (for Mac)
Alternatives on How to backup Xperias
http://talk.sonymobile.com/thread/36355
Hey I will try charging it for 2-3 hours now and see if that does anything. I took out the sim and MicroSD card last night. Thank you.
Ok so I left it on charge for 4 hours. I cannot connect to PC companion still. It will turn on but just loops the initial Sony Xperia screen, turns off, back to that screen. When I plug the USB cable to a computer or to power I get a single red LED flash and it turns off.
It's screwed basically. If anyone knows how to do a factory reset or something that would be amazing.
@eubie12 I'm glad to hear that! 
@RedWater Have you tried the software repair? How far in the process do you get, all they way until you are prompted to connect the phone while pressing and holding the volume down (-) key?
Click here for more detailed instructions on how to perform a software repair: software repair using PC Companion.
I cannot do the repair as I cannot connect to anything. As soon as I plug the USB cable in and connect to the PC or a wall charger the phone crashes. It will not connect to the PC companion. At all 