Temperature guage in vw polo not working?!?

my car out of the blue yesterday overheated very quicky, went to the garage and steam and water everwhere. a new thermostat was fitted i believe and when i left the garage took the car for a run and the temp guage didnt move at all also the heating inside seemed to be nowhere near as hot as normal, although would blow cold and warm air. any ideas why the temp guage isnt working?