Check your phone with another compatible sim. If that doesn't work then your only other option is to send it to your nearest Nokia Care Point.
You can find your nearest Nokia Care Point at:
http://www.nokia.com select your country, go to the support section, then select repair.
i m facing similar problems.. last time i had this problem i restored it to factory setting. it worked fine for some days, but suddenly today it again crashed..
i updated its software too but no help..
kindly tell me the solution asap...
If i have helped at all a click on the white star below would be nice thanks.