Before doing anything else check the coolant level in the radiator to be sure you have enough coolant.
With the ighition off check to see if the guage goes to zero. Then turn the ign. Switch on (but do not start the engine) to see if the guage moves at all (if so guage is working).
Check for 5 volts at the coolant temperature sensor (located just below where the top radiator hose connects to the thermostat housing) it has an orange/black wire going to it. Unplug it to see it 5 volts are present. If so, momentarily (split second) ground the wire while a partner watches the guage to see if it "jumps" if so the guage is working. Next suspect the sensor as being bad.