Just use the "Explorer" function under "Drive & Devices" in the left Navigation bar, with the "Show Only Files In Library" checkbox ticked to navigate to where the files are, and then fix their [Media Type] and [Media Sub Type] tags as required.
Note that there isn't actually a [Media Sub Type] of "Video", so I assume you had [Media Type] set to "Audio" and changed the [Media Sub Type] to "Music Video". Then if your View excluded files with a [Media Sub Type] of "Music Video", the file disappeared from the View.
If the above doesn't work, could you check and clarify what you actually did?
