If your hard disk drive platters have been damaged for any reason, unfortunately, there is no way to repair it because it is hardware damage. However, before concluding that it really is hardware damage, it is still important to do troubleshooting by testing your hard disk drive to another computer system.
Many times over voltage will NOT damage the actual workings of the hard disk but rather the PCB. There are a set of protection resistors and diodes that will generally burn out before any really serious damage happens. If this is the case it is a DIY repair that you can do to salvage the the data on your drive. It is highly recommended that you not use this drive (except to backup the data on it) after bypassing the the resistor and diode as the drive will no longer have over voltage protection and even a small power fluctuation from the PSU will burn out the drive.