What does this mean?

As one of the more likely things that might cause this -- based on your having left the car without running it a period of time, it may be the 12v battery.

Easy enough to test -- just google it. Lots of threads on here about how to test, as well as lots of youtube vids.

How old and what brand of 12v do you have installed?

 

You're welcome. (sounds like your 12v may be weak, not totally dead. You could try recharging it, depending on it's age and it's general condition).

 

If the vehicle is "ready" (image shows such) then the current 12v is most likely fine (for the moment), and you have other issues at hand.

Find a toyota hybrid compatible OBD2 code reader, scan or retrieve the stored codes (DTCs), then post them here for further guidance. Most autoparts chains offer an OBD2 scan for free. Not all scanners are compatible, try other stores, even within the same chain. You may need a mini-vci cable and techstream (same software the dealership uses).

How many hours per week do you drive this vehicle, and how many miles are on the odometer?

Have you had a chance to look up the warning lights in one of the guides found inside the vehicle or from this link here?

 

it means: oh boy

 

I disagree -- which I cannot believe -- with SFO because SFO is really on target all the time.

The dash pix looks like mine did when my 12v was "starting" the car, but was reading 11.2 v.

The key clue given was the car had not been driven for "some indeterminate amount of time."

Quick enough to rule out the 12v and I really think that is the first thing I would check.

 

There is no reason why the car shouldn't start with 11.2 V on the aux battery. The is something else going on.

That clue is more likely to point to the HV (hybrid vehicle) battery, TBH.

 

This is true. But his battery could be low enough to make various codes appear. It only takes a few seconds to find out. Then read the codes, which I think is really most likely.

It means you need to get the codes read by a code reader that speaks Prius. It won't hurt to check the charge on the 12V. It's a possibility and it's low hanging fruit. But given the car's age, something in the traction battery is more likely.