If the above suggestion doesn't work you probably only have three options here.
1) Soft reset -> volume down and power button together, hold until it vibrates, then let go. Phone restarts. You may need to reset date and time.
2) Hard reset -> Settings -> About -> Reset phone (note this erases everything on the phone and there is no way to backup game data unless the game allows it) Be sure to backup before doing this.
3) Use NSU (Nokia Software Updater) Download from here:
http://forums.windowscentral.com/wi...ng-gears-cogs-bug-nokia-lumia-users-only.html (ignore the title you just want the software)
With NSU you need a WiFi connection and a Windows 7 or 8 computer for it to work. It will download a new firmware for your phone and completely wipe the other one off. So it's like reinstalling WP OS.
I've noted that people get phones from all over the place so if your phone is a different region to the country you're in be sure to set the region on the phone once you do a hard reset or use NSU. Otherwise your apps won't download and the SIM won't work.