You need to sort out your 12V problem. It will not charge or do anything without the 12V circuitry working, the OBD-II is also all 12V. The service manual shows how a mechanic can release the electric parking brake by removing the motors from the calipers and winding the piston in by hand with an allen key. It's probably easier to unplug the connections and use a jump start battery to do the winding, but I wouldn't go there to start with, I'd be looking at getting the 12V circuits up and running.
I strongly suspect that it's just your 12V battery that's died if you drove it there and everything was working fine with no issues, and then in the morning it's dead. You could probably just change the 12V battery and it would be fine.
I know you say you tried jump starting it, but I wonder if maybe the battery is too dead or you didn't get a good connection with the jump leads? I have a voltmeter and I'd first use that to see if there are any volts on the 12V battery terminals. A simple circuit tester light is going to need a reasonable voltage to show anything, where a voltmeter can be far more sensitive. If > 12V then it's a fuse that's gone or a bad connection. If < 12V then the battery is dead - question then is did it die of age or was there a short on the car that killed it? There is a big fuse (I think something like 150A) on the side of the underhood fuse box where the battery cable connects in to it. In that fusebox are the main fuses and relays that power up the rest of the car (IG1 etc). Also remember to trace through the negative cable and measure the voltage between the battery negative terminal and the chassis - poor gound connections can cause all kinds of weird symptoms.