I also have a Tempstar Counterflow Gas furnace that only blows cold air. This is the information I got off the tag:
A-1988
Direct Blower Drive
Model # 867.766142
Serial # L904639819

I've tried turning the thermostat off and then on again but this has not changed.
It was working fine 10 days ago.

Does the blower run constantly unless you kill power to the furnace? If so, check all high limits to make sure they are closed.