if your heaters were blowing hot then i doubt its the thermostate because if the stat was stuck shut not allowing the water to get through then the heater matrix will not be getting any hot water and therefore would blow cold air. it may be as simple as the temp sender is fucked or the wire from the sender has shorted to earth. if the wire has shorted to earth the temp gauge will go straight to the red with just the ignition on and the engine not running.
but on the bad side it could be head gasket!!!
hope this helps,

cheers,
Phil