I had exactly the same problem a few days ago.....I went to play with my new toy but alas nothing happened. Tried to speak to the so called technical Help on the sony mobile site....About as much use as a chocolate teapot!!!!
Any way aftermuch panicking and trawling the tinterweb for advice....still no joy
Then I had my eureka moment and remembered back to my Iphone days - (sorry for using the forbidden word!!!!)
I thought what would happen if you hold all buttons down????
After around 6 seconds the phone vibrated
I then pressed the power button and as if by magic the phone was back up and running with nothing effected
(this meant the wife was happy as well!!!!)
I think this is some sort of jump start/ reboot for the phone but since then it has been working fine!!
Try it and let me know