Of course, you used the MC Rename, Move, and Copy Files function to move the files, didn't you?
https://wiki.jriver.com/index.php/Moving_FilesIf you moved the files outside MC, say using Windows Explorer, and you aren't using or didn't move the video Sidecar files, then MC considered all the moved files as new files and re-imported them... without any of your tags metadata, because they can't be saved in video files. If you were allowing video Sidecar files, and you moved those as well, then MC would still have re-imported them, but it would have found and used any tags in the Sidecar files. Of course, not all fields in the MC Library as saved as tags in file or Sidecar files, so you would still have lost some metadata.
While you could do some stuff to fix what you have done, the easiest (if not the fastest) thing to do is move the files back to where they were, and then use the RM&CF function to move them again. All tags will move with the files then.
If the above doesn't make sense, or you deleted the original directories on the source disk that still held Sidecar files, explain
exactly the steps you took to move the files.