I had bought nokia E71 from nokia store and now after 1 odd month it has started showing sim card registration failed followed by insert sim card message on white screen...what could be the problem and i have already checked that sim card is ok...

Sim card registration failure means that the card in the phone is not being recognized by the service provider as a valid or current account. The service provider is denying service based on the sim card that you are trying to use in your phone. If you have a current and valid account with your cell phone provider then you need to contact them to remedy the problem because something in the registration process has failed. They need to update and/or correct their records to link your account to the sim card in your phone so that your service will work.