Turn off the phone - press and hold the power button until you see a slider to turn off.
Take out the SIM card from the side. Rub it with a microfiber cloth and reinsert it.
Turn phone back on.
If that doesn't work, Go to Settings and then click General. Scroll to bottom and click Reset. On the next page select Reset Network Settings. This will not change any personal data on your phone. The phone will reboot. See if that works. Any wifi passwords will be wiped with this reset so you will need to set those up again.