How do I fix my Nokia N95?

My Nokia N95, was working fine, then it started to show SIM card registration failed, this happened within 2hours of receiving and sending a text message. Phone has NOT been near water, and SIM card works in another phone. I have switched phone on and off and checked SIM card in another phone so the problem is not with the sim card. Everything else on the phone still works, camera, music, clock and alarm. Can anyone help me please?

Fiona
June 2011