Lost a track due to no association

I was trying to tag an album using MusicBrainz. I found a version that had one track less (11 tracks) than what I had (12 tracks). I assumed that I could still use the tag information and just have in place of the extra track to edit manually. Unfortunately it seems that by using that method deleted the track that had no association. It didn't move the *.mp3 file to trash, it's just... gone. Where has this file gone and how do I get it back?

Usually, using a web source script does not delete files.
If there is no correspondend entry in the web data, then the file simply does not get tagged.
So the file should still be there somewhere.
Have you applied a filter? Best switch it off (F3) and see if the file reappears.
Press F5 to reload all files (just in case you have pressed the Delete key which removes a files from the file list but not from the file system).
So, keep looking.