First, make sure all your possible loads (other than the BMS) are disconnected and get the battery warmed up, if you end up having to disconnect or bypass your BMS for a bit, you'll have no under temp charge protection. Sounds like you're already heating the cabin, but I'd hesitate to go much farther until you've measured the temperature of your cells above about 5 Celsius.
Second, measure the voltage of the battery at the cells. i.e, directly measure the voltage from the negative of cell 1 to the positive of cell 16. Note that this could be different from what you measure in lots of other places in your system that would normally all show the same voltage... If your BMS shut down, everything downstream of it can show strange voltages; not necessarily zero, but also not pack voltage.
If your measurement above is still 12-13 volts, start measuring individual cell voltages at the terminals. If some (or many) cells are very low or zero, there may not be much you can do to save them. Maybe there are others on the forum would have some ideas on that.
If your measurement in step 2 above is more reasonable but still low, say ~40V, you may just be in a situation where your BMS did its job and disconnected rather than killing your pack. It would probably still be worthwhile to measure the voltage of each cell to make sure there are no big outliers. If there aren't, jump your 9V to start the BMS. If cell voltages are still below the Power Off Voltage, it should just shut right back down, but I'm not sure how much (if any) delay there would be before that happens. If there's a delay, you could quickly go into your JK App and turn on emergency mode to keep the BMS running and then start recharging the battery, but note that in emergency mode all the normal protections are shut off, so monitor closely. If there's no delay on the BMS shutting down but your multimeter shows that your cells aren't completely ruined, you may have little choice but to bypass the BMS to get some initial charge into the cells. Do so with great caution.
As for how to charge, my Inverter/Charger will run on AC power (e.g., a generator) and just complain about not having a battery connected, but it's not a Multiplus, so I'm not sure if yours will do that. It may need battery voltage to start.
There's already a few diverging paths here, so maybe report back some results, like pack voltage, cell voltage, etc., and we'll see where to go after that.