If the touch screen doesn't work on your phone I suggest that you first of all try to restart the phone by press and hold the power button and volume up at the same time until the phone vibrates.
If the touch still doesn't work you can repair the phone software using PC Companion:
http://www.sonymobile.com/update
In PC Companion, press start on Support Zone > Start on Update the phones software > Press the blue link that says "Repair phone".
Before repairing the software in your device you may want to backup your information first. Check out this topic for more information on how to:
http://talk.sonymobile.com/t5/FAQ/Backup-amp-Transfer/m-p/407253#U407253
