I have had same problem with my sony xperia T2 ULTRA. I tried everything from format to factory settings to restarting from SIM tray reset button but did not work.
Finally I opened lid and poked the button and and mechanism from inside of phone softlywith a needle then it turned to normal. Button is losing it's function or sticking to area where power comes then not working properly.
This is the problem.
Now it is working fortunealty.