Find the blue coolant temp sensor on the engine (you'll need to remove the engine cover).
Check it has voltage in.
I can't remember if it has two wires on the plug but if it does bridge the two and the gauge should go all the way to the top.
If it is only one wire connect this to a light bulb and earth the bulb, it should light and the gauge should move.
If the gauge moves the sensor has failed, if it doesn't the wiring is broken somewhere.
Home to black Alfa Romeo 159 3.2 V6 Q4 ,green MGF VVC and red MG Maestro T16.
MG - the friendly marque.