Key was in ignition - believe was left running - despite charge no power of any kind

My deepest condolences to you... Hooking up a different 12v battery via the jump start instructions in manual would be a good first step. Trickle chargers take a very long time to fully charge a 12v so the 100% reading might be in error.

Are there any other symptoms or details of what you can tell happened with the car when this very difficult moment happened? Do you think it ran out of gas?

Once you get some power in the car, there might be warning lights and you'll be able to read the stored error codes that triggered the lights, which will point to specific details of the problem.

 

My condolences

Not sure if there is a auto shut off other than running out of gas.
Not sure what would happen in that case, but quite possible that it ran down the hybrid battery all the way.

You may get lucky and be able to disconnect the 12v battery for a few minutes and then reconnect.
But perhaps if that is low, you may need to trickle charge it or jump from the jump
points under the hood. Under the fuse box on the drivers side, flip the RED thingy over,
connect the + RED and then - Black to perhaps a metal piece of the engine for ground.
But being this is a Prius, be very careful to not cross them, or cross the other side of the jumper cables etc.

In the event that the Hybrid battery fully ran down to danger territory,
you may only have the option of bringing to a Toyota dealer to charge up to safe levels that battery.

But try out the 12v and see if codes appear or if everything is all good from there.

 

You have my deepest sympathies.

A 4+ year old 12V battery taken to deep discharge in combination with typical Alaskan temperatures for mid-December can permanently ruin the 12V battery. It may not be able to accumulate a meaningful charge from any charger at this point.

The setup you've described could have turned into a cascading failure as follows:

Car left running, engine recharges the big high voltage (HV) battery as needed, cycling on and off. The DC/DC converter in the hybrid power system produces low voltage power to top-up the 12V battery much the way a traditional alternator does in your Ford.

Eventually the fuel runs out and the engine stalls. The hybrid computer doesn't have the greatest awareness that the problem is fuel exhaustion, so it will try to start the engine again. Starting power comes from the HV hybrid battery, not the 12V. However, the 12V battery is carrying the load of several ECUs plus any 12V accessories that may have been left on.

After X attempts to start the engine, the HV battery will have run itself down. The 12V battery is still operating the ECUs and hotel loads.

With the HV battery exhausted, the DC/DC converter can no longer replenish the 12V battery. The ECU load draws that battery down.

So, to get back up from this situation the first thing is to get 12V power back up. Connecting another 12V battery via jumper cables is a good start. That lets you get the electronics booted and see if it's got trouble codes logged. If it really was run down, recently-set codes are likely lost. However it will start generating new ones if it attempts to start the gas engine and can't due to an exhausted HV battery.

The bad news is that a discharged HV battery needs to be mains-charged before it has enough power to crank the engine. It's a really unusual charger. Only Toyota dealers have them. Not all dealers have it; sometimes one must be brought in from another dealer or regional hub.

I'm not there anymore, but I grew up there. I know it can be a tough season. Stick close to anyone you can.

 

Everything Leadfoot said above is correct... But if you do DIY work on cars you got this... YouTube videos will show you how to pull out the hybrid battery and then you can remove the connectors and charge up the individual 8 volt modules... There are 20 of those modules and you will need a hobby charger that's designed to charge up remote control toy cars, planes and boat NiMH batteries. These chargers can be bought online for $30 - $50, I can send you links to the best ones if you want.

The other more expensive option, but easier because you don't have to take the battery pack out of the car or take the car apart very much is buying a charger and wiring harness in the $400 range here: Prolong® Battery Charger - Hybrid Automotive

Hopefully the problem is just a 12v battery failure due to cold weather and the hybrid battery still has enough charge to start the engine.

 

Glad to hear you've got the vehicle recovered properly. I think it's wise to replace that battery after the discharge/freeze combo.

Thanks for the picture! Your car looks great. I can't place the setting; been a long time since I left. My first job ever was delivering the News-Minus.

I wish I could suggest a solution for a wiring path. So far the only thing like it I've done on ours was a 4-wire trailer lighting setup, but obviously that's the back of the car.

 

Congrats on it being less of a hassle than it could of been...

When I run wires, I look for an existing rubber grommet that wires are going through in the firewall and run it with those wires, or poke a hole in the grommet next to those wires...

 

If it was a Gen2 Prius I would, but I've yet to work on a Prius C....

 

Gotcha, I've only been on base twice, airshows etc. Still nice to see a pic from "home."

I'm not near our c today, but it occurred to me that both the main underhood relay box and cabin fuse panel are at the extreme left side of the firewall. I'd be very surprised if the wiring between them didn't transit the firewall in the same plane.

You can rent the real-deal Toyota service manual for $20 and save the PDFs for future use. It helped me a lot when I added a carplay stereo and a dashcam to ours.