Sounds like an electrical problem more than a cooling system issue.....
Was the gauge 'pegged'? I'm always suspect of pegged gauges when I know everything *should* be okay.
Was the engine insanely hot? The fact that there was no hot air coming out the defrost vents... . could mean a few things:
1) The engine was stone cold - not like Steve Austin... . and the gauge/wiring was just messed up.
2) There was not enough coolant to fill the heater core - and the engine really was overheating.
Let us know a little more about the problem... .
Matt