1995 Ford Taurus Coolant not circulating, no heat

Car had episode of coolant loss and overheat. Car has very little heat--barely warm at best. Fluid in reservior is cold after long drive, nothing seemed to be circulating. I replaced thermostat after overheat. Still no heat and temp gauge does not move, stays on the lowest possible (I'm not sure where the sensor is, my grand am was in the reservoir, so if the Taurus is, then it's no wonder it's low).
Took to dealer, they replaced the thermostat--and the other was only a week old. Gauge is still on cold, heat is still only warm, reservoir is still cold. Took back and had them look at it--they looked very confused. Does not seem to be a head gasket at this time. Any ideas? If it turns out to be a head gasket--how much do you think?