what happened ?,,thats hard to say.
the guage could be malfunctioning,,there could be a loose wiring connection,,the coolant temperature sensor could be faulty,,,or a fuse could have blown,,, or the ecm could have stored a trouble fault.
also the radiator fan relay and radiator fan could be faulty.
the first thing to check would be the junction box for a blown fuse,,your owners manual will indicate where this is located at.
the next item to check would be to have the ecm scanned for any stored or active trouble codes.
following with a check of the relay,sensor,,fan and wiring and connections.