You might need to do one of two things:
A. Look in every major view in the files section: Audio, Video, Images, Notes, etc.
B. Build a new top level view that can see everything. See, the Audio and Video views are limited to their specific types. Audio can only see files with [Media Type] = Audio. Video can only see files with [Media Type]=Video. I have a view called "Files (everything)" that is unlimited. It doesn't happen every often, but EVERY now and then I get a file that gets tagged funny. Like [Media Type]=Data and then I can't find it. Until I go to my special view.
It's really easy to set up a new view. Just add a new view and put in in the Root. Make it a Panes view to make it simple. By default it should see everything in your library.
Good luck!
Brian.