Ended up being the float sensor, which turned off the HVAC, which then caused the nest to drain down to nothing. We pulled the Nest off the wall, charged it via USB, plugged it back in, read the errors and called an HVAC guy (we don’t have a shop vac). He cleaned out the float sensor and the drain and bam, things were working again.

Would have been brutal to come back from Disney, yesterday, to a house that was 80+ inside.