Hi
It certainly sounds like the hard drive electronics have been fried but the information could probably be recovered from the platters by a recovery company but expect a high cost.
if you can source an identical hard drive you could swap the PCB and that should allow the recovery of your data.
There is a chance that only your logic board is burnt and that the rest of the drive is still functional.But You can't fix the logic board unless you want to learn data recovery and possess SMT soldering knowledge so it's might be time for a new Hard drive.
It sounds like this happened with an external hard drive? Using the wrong power brick? If that is true then the enclosure is toast but the HDD contained in the enclosure may (or may not) still be usable. It will need to be removed from the enclosure and hooked up to a computer or put in another enclosure to test. If you aren't skilled in these things I'd suggest bringing it to a computer shop. They should be able to tell you very quickly whether it spins and works. And if it's dead they can suggest data recovery services (very expensive) if the data is absolutely precious. Good luck!
In most cases the result of such an overvoltage is a shorted 12V TVS diode. Depending on the brand of HDD, there may be an easy, zero-cost DIY fix. Otherwise a PCB swap plus firmware transfer should cost no more than US$50.