Try to start the phone again by press and hold the power button and the home button at the same time for about 10 seconds.
If the phone still doesn't start, try with another SIM card. If that doesn't help I suggest that you try to 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"
