Red Triangle of Death and Engine not starting, but no OBDII error code.

I have a 2008 Prius, about 190k miles on it. No warnings ahead of time and nothing that made me concerned. One day I hopped in and the car just wasn't working. Inverter pump was replaced about two years ago, and I've kept it in pretty good condition.

My car's battery is activating but the engine will not go on (I can't shift out of park), and the Red Triangle of Death is a glow. Googling/Prius forums suggested that this could be a bunch of things with the common three being a weak 12V battery, a busted inverter, or the High Voltage battery being kaput.

I tried to jump start the car, no luck. Most advice then said scan to get the error code and then clear it to start the car. I used an old OBDII reader I borrowed, got no codes. Thought it was because it was out of date so I bought a new OBDII reader (the Ancel AD310) but still get no joy.

Anyone have advice?