12V battery died, now hybrid system is not working

your best bet is to get a prius aware scanner, and post the trouble codes here.

 

Has the high-voltage battery or its individual cells been replaced before?
Has the 12V battery been replaced before?

 

The red triangle may indicate problems in the hybrid part, including the high-voltage battery. There may be a discharge of some of the 28 modules that create the high-voltage package. To accurately determine the cause, you need to read the errors with a scanner.

 

The OP's location is "Lincoln", as in Nebraska? Did the car maybe sit for 3 weeks at subzero temperatures? That can certainly kill a marginal 12V. And it does sound like the 12V because it would start with a 12V jump, but not without one. A (conventional) jump won't do a thing to the HV pack.

The dash light issue might just be that the capacitor problem in the display (a common and well known issue) happened to kick in. Maybe marginal capacitors don't like to be frozen for weeks either. Then it was reporting 16 MPG after the new 12V battery. Well, what temperature was it? Fuel efficiency isn't great driving slow with a cold gas motor in snow at -10 F. What did the energy display show? Was the HV low, like just a bar or two, with the ICE charging it? That also will lower MPG.

Finally, red triangle of death. Get an OBD 2 scanner which can read all the codes on this car, or the proper adapter and a copy of Techstream, and read the codes. That will tell you all the problems the car itself is aware of.