I ran a disk explorer type program, the files are indeed just zeroes now, never had anything like that happen before.
I once had a fiasco that took this to a higher level.
It was back in the days of floppy disks and I was doing some assembly language programming. While testing my program there was some unexpected drive activity which lasted several seconds.
After that I couldn't see any files on the floppy disk. A sector editor program showed that the File Allocation Table consisted entirely of zeroes. All the data was still on the disk, but totally worthless. With no information on how to match up sectors with files the data is just a meaningless jumble.