The FAT / FAT32 file system, as used on flash memory, is not that robust and is easily corrupted. Unfortunately, Windows’ solution to a corrupt FAT or FAT32 file system is to format the drive, which obviously wipes it blank. This issue is usually caused when the flash drive is ejected when the PC is still writing to the drive. Even if photos were being viewed, windows automatically writes a hidden thumbs.db to the flash drive, so it is always advisable to do a safe hardware removal or Eject operation before removing the flash card.
Going by the issue you’ve encountered, the file system’s directory structure is damaged to the point where the filenames can no longer be read. So what most recovery tools do is do a RAW file recovery. This means that it physically scans the drive for recognisable file headers to extract and reconstruct the files. As the directory structure appears to have been damaged, the utility cannot retrieve the original file names and directory structure, so it gives them all generic names, e.g. “Recovered_0001.jpg”, “Recovered_0002.jpg”, etc. depending on the utility.
Another utility worth trying is PhotoRec, which is a freely available tool specifically designed for recovering photos from damaged flash drives also.