As Jim says, odds are a stuck (open) thermostat. If you stop and restart the car the gauge will start reading again. If the temperature doesn't get up to and stay in normal range within a certain time, the fault monitoring in the ECU detects this and turns the gauge off, and the EML on and also the radiator fans on low.
There's an outside chance it may be the sensor. How to confirm diagnosis- take the car for a drive until it's warmed up. The gauge may not get up to normal temp or it may fail again. Stop the car, park it up, restart and run at a fast idle for a few minutes with the aircon off. If the gauge gets to horizontal, it's almost certainly the stat.
However given that both are cheap, I'd replace the stat and sensor. If the sensor is reading low, your fuel economy will be suffering.
