There are file protection algorithms in ZBrush to prevent a corrupted file from overwriting a previous version of the file. However, there is one unavoidable vulnerability in that feature: If the power goes out while the final write is taking place then only a partial save will have happened and the file will be corrupted.
It’s a one in a million (at least!) chance that this will happen but there’s absolutely no way to avoid it – short of having a UPS to provide battery backup, anyway. The bottom line is that the file was only partially written because the power outage interrupted the process. It can’t be recovered.