Frozen & drained HV Battery

The Battery ECU has some programming to limit or eliminate use of the battery in extremely cold temperatures. So it might be fine once the weather warms up.

Alternatively, you can use a high voltage trickle charger to recharge it. Maxx Volts and Prolong make them but they're expensive. You can also build your own for $100: Build Hybrid Battery Maintenance Gear For Under $100 | PriusChat

Or this guy near you in Madison might loan you one: EV Powers | Hybrid Battery Service & Repair. Prius Insight Civic. 56 Corry St, Madison, WI

 

This sounds about right to me. The battery pack isn't likely to freeze but it might not work very well until the car has warmed up. That doesn't mean that the pack might not have failed just from old age.

The OP says that the 12V looks OK, but did not provide a voltage. 12V batteries often fail when exposed to low temperatures. I had one drop dead in San Jose after a single cold night (around freezing), despite not having shown any signs of weakness previously.

If the OP has an electric room heater and a long extension cord it might be interesting to warm up the inside of the car (very carefully, don't want to burn it down!) to at least 40F, for an hour. Pull up the rear tray and the cover over the 12V so that it can warm up too. Then measure the voltage on the 12V, and if that is good, try to start it. If it still has a red triangle get a code reader on it and see what that says.

 

Yep... We're still in the early days of batteries in cars. Systems that maintain your cells at a precise temperature and even precise humidity and air pressure is something that Tesla and EV makers are realizing giant performance improvements from.

Someday soon the necessity of preheating and pressurizing your battery pack will be as normal and as expected as turn indicators and headights.