Mikepier writes:
Make sure your drain tubes leading outside are not frozen.

Interesting. The condensate tubes themselves are fine (and inside) and
in fact are still draining a steady flow of condensate.

If you are referring to the intake/exhaust tubes for the high efficiency
burner that is possible I guess but seems unlikely given that the
weather wasn't too cold, the pipes are relatively large & short, and
there is no really good way for water to get in them (they are above the
snowline).