xperia u doesn;t turn on

[Sim_21]

I am trying to on my phone by using power button but once it vibrates it flashes sony then xperia then sony and then xperia. I can't even turn it off. Nothing happens after that. I tried to connect to pc campanion but it doesn't even recognise my phone even after removing battery and pressing power+volume up as it starts in the same manner again flashing xperia sony xperia sony again and again. please help me out. 

[Thommo]

Sounds like you need to perform a system repair which will factory reset your phone

Switch off your phone and unplug from Pc (Hold volume up and power for around 10 seconds)

Start PC Companion and select Support zone then Update my phone/tablet then in Blue Repair my phone/tablet and follow the onscreen instructions - When prompted connect your phone still switched off holding volume down or back button - This should start the repair or reformat process

If you are using Windows 8/8.1 or a 64bit operating system then adjust the settings for PC Companion and run in compatibility mode and chose Windows 7 or XP

[Sim_21]

pressing power and volume up button doesnt work instead phone keeps on flashing sony xperia sony xperia and nothing happens.

[Thommo]

Then you need to keep them held down for longer untill your phone fully powers off