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Followed onscreen instructions during latest update using my laptop as this update can only be done using your computer. The m2 switched off as per screen instructions then when I followed the next instructions the mobile would not respond it looks like it is dead, it will not switch on, so the update is not complete, so I am now with out a phone thanks to Sony and am not a happy person as it is 21.55pm and sony customer support does not open untill 8am
You may need to perform a software repair using PC Companion.The process will erase all internal memory.You might need to charge your device for about 1-2 hours before so the device has at least 80% battery.
Make sure the device is not connected to computer or laptop and turned off (sometimes you have to press power button and volume up until the device vibrates 3 times or on some devices you have to press the small button near the SIM tray or on the back of the device).
Open PC companion->support zone->phone/tablet software update->repair my phone/tablet blue highlighted.Follow the steps given,do not connect your phone until prompted to press volume down while plugging USB cable. If PC companion does not start the repair process, you might need to run it in compatibility mode with Windows 7 (in some cases XP).
You might want to contact your local Xperia Care
The phone doesn't need to be turned on for the PCC repair. But maybe it would be wise to follow uli's advice.