If you put 12V through the 5V line then it is likely you have killed every single chip on the board. The 5v rail goes virtually everywhere, most TTLs, RAM and ROMs will die at 6V and some Z80s are ok up to 6.5. Once you get beyond 7v everything pretty much dies. The pop you heard is either a chip blowing out on the underside, or something shorted out internally and connected the 12V to ground via a PCB track. The PSU will step up to the challenge and provide an amp on the 12V line which will vapourise whatever track is in the way. Chances of it being a complete and total wreck are about 99% at this stage.