Red triangle and engine will not start only battery running

First, don't drive it this way.

There's no telling what's going on without having someone read error codes from the system. But rather than a dealer, perhaps you might search or ask around for a local independent shop with Prius expertise. You often get better and more affordable service that way.

 

Maybe some more learned Prius Chat brethren will have some idea.
But basically your almost 18 year old Prius, is telling you by virtue of the Red Triangle and it's operation that something pretty serious is happening.
If I had designs on driving for many more years, or many more miles beyond whatever this incident turns out to be? I would stop driving it and take it to a dealership or hybrid shop. Tow if necessary.
I don't underestimate the brilliance of many of the contributors here. But basically you have something happening, the vehicle isn't running correctly or safely, and the only real way to know...is to get it checked out.
Good Luck.

 

You got to read the codes. However Toyota did not make this easy to do. You need a OB2 scan tool that can read that special HV codes Prius has. Most common OBD2 readers dont do this well or at all. Cheapies usually just say "no codes present" even though you have lots of warning lights on. First check your 12v battery, get it load tested at a competent shop. Just putting a voltmeter on it is not good enough.

In any case I will bet you dollars to donuts your issue is inside the hybrid battery pack. The most common trouble source is failing modules. You can replace bad ones with refurbished used modules at places like ebay and other hybrid car parts vendors. I bought what I needed year for about $20 each. I needed 18 of them for my 2002, ouch! My car runs great again. Of course if you are not very DIY then a refurb pack may be better in your case. $1200 and up.