You should only loose the data that was currently being written to a new space on the drive, not the drive itself.
If that data was master boot record then a full wipe and reformat of drive is require to get it going again.
If windows system file then anything from relacing the file via sfc or a complete reinstall of the op system.
If important data then bad luck its gone.
I don't think any commercial defrag progs would be better in the case on mains failure.