Try removing your battery out of your mobile and let it there for ten 10 or 15 minutes and again put it and turn on the phone. This error is coming because your number is properly registered to your service provider. So when you turn on again the registration process will start again and if still error comes up please inform their customer care department and they will do the needful.

There is another soltion to the problem your phone was shut down and is no longer useable unless you turn it back on