HOLD light flashing, engine temperature gauge "C"

I don't think the clock resetting is related to this, although it could be a symptom of a computer problem. The possibilities that come to mind are:

1. Sensor failure: the temperature sensor may have failed and be giving a low reading
2. Sensor wiring problem: this is fairly likely, it all comes down to how electronic temperature sensors (i.e. thermocouples) work: they offer less resistance to electrical current as temperature goes up, i.e. as it gets hotter the voltage on the circuit goes up. Themrocouples are calibrated to give specific voltages at specific temperatures, the temperature readout is simply an indication of the voltage on the circuit. The gauge is at its lowest possible reading, so if you know it isn't right that indicates there is no voltage on the circuit, indicating a break in the wiring or loose connection
3. Computer failure: the car's computer(s) may have a problem and be misreading the sensor, or displaying the sensor incorrectly
4. There could be a problem in the instrument cluster electronics

I suggest using an OBD reader to check the engine's temperature as reported by the computer. I use a bluetooth unit with torque on an android tablet personally, but there are many options available. If the car's computer reports a low temperature then it's likely the sensor, if it is giving a P0117 code (low voltage on temperature sensor) then it could be a failed sensor or the wiring.

If OBD shows the temperature is realistic then you have another problem, for instance 3 or 4 above.