Share your experience!

Have you tried a software repair on 4.3?
Either Pc companion or Sony update service:
http://www.sonymobile.com/gb/tools/update-service/
I am sorry,did you try another SIM?
Yes,it is a simple guess.Remove the SIM and see if the phone restarts.
You can contact xperia care:
http://www.sonymobile.com/global-en/support/contact-us/
And I was not suggesting using the phone without the SIM,just see if it works without your SIM.Maybe it is broken.