Hold down on/off button and volume up until you feel a vibration (phone will restart)
Hold down on/off button and volume up until you feel 3 vibrations (phone will turn off)
Does anything happen?
Also check that you have charged the phone for 3-4 hours and then you can try again.
If there is a physical problem with the button I recommend you to contact your local support for further information on how to send it in to a repair center for investigation:
http://www.sonymobile.com/global-en/support/contact-us/contact-info/
