shut down with no reason

[NataliaMonte]

I tried to unlock my phone, but it didn't work. Instead, the screen stood off, and the phone did't respond to any command. I pressed the reset button, and tried to turn my mobile on, but it just didn't work. The battery was okay, but, I tried to charge the phone, and the little light from the bottom turned on, but the screen kept off. Please, somebody help me! 

[Uliwooly]

connect the phone to a wall charger > press and hold the power & volume up for 10 seconds > let it charge for 2 or 3 hours 

[_alexdon_]

You might want to consider about a software repair:

http://www.sonymobile.com/gb/tools/update-service/