Well if I understood correctly, that's what happened. 12V to 5V so most probably the CF card is dead. I wouldn't be so sure about the miggy though. They're resilient fokers

First of all check the CF card in a CF reader an in a PC (And check in disk management if the PC recognise the card physicaly)
Second, check via correct powering the miggy, (without CF adapter mounted) if Amiga starts up.
One last easy step, is check with a multimeter in continuity test, the +5/GND and +12/GND and +5V/+12V. If it beeps... then it's bad

Maybe people with more experience can help you better mate
Good luck