First of all, power the device off, then back on. If that doesn't resolve the issue, try resetting network settings (Settings > General > Reset). If all else fails, restore the phone again.
If that still doesn't resolve the issue, you'll need to restore the phone "as new" when prompted in iTunes at the end of the restore process. This will completely erase everything on your phone, and delete your backup, so you'll need to have all of your info (contacts, calendars, emails, pictures, texts, etc.) saved elsewhere.
Good luck. 🙂