It is my understanding that the HV battery can be recharged, minimally recharged, using a fully charged 12 volt battery as a charging source. My understanding is that there is a DC-DC voltage up-converter (inverter) for doing this. Might take ~24 hours to restore a ~full charge so it would probably be a requirement to have a charger on the 12 volt battery during that entire period.

Obviously there is a possibility that your Dc-DC upconverter has failed.