I suggest that you try to clear data for the messaging application. Go to settings > apps > all > messaging > clear data. Restart phone and try again.
If the problem still persists, try downloading another SMS application like Go SMS or Handcent SMS. If you can access the messaging database using that app. Try to delete some conversations and restart the phone, try the ordinary application again.
If that doesn't help it sound like you need to update/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"
