Now that it has warmed up, things are back to normal. Even my heat is back. I think that my thermostat was stuck, but can they get "unstuck"?
It depends, it could have become stuck (frozen) but should have free'd up after your engine was running for a while. In extremely cold temperatures the airflow through the engine bay is enough to cool the engine so it never warms properly. In some places its normal to cover the radiator to block some of the airflow and help keep the engine warmer.
I know some newer ultra efficient small capacity diesel cars (Ford Focus) have automatic grills which block the airflow until the engine is up to temperature to aid in efficiency but I would imagine they also help in extreme cold.