Device issues cannot always be fixed, but if it could be fixed...it would be by one of the following methods:
Pull the battery for 30 seconds.
Put the battery back and try again.
If it is now resolved, it was just a start up issue.
If that didn't work, call your wireless carrier. Do all troubleshooting their. They'll assume it's the network and have a tech look at the network deeper. If the tech doesn't find anything, then something is wrong with your phone. You need a replacement.