go into your itune preferences and then go to advanced ( i think i am on mac and i am just guessing your on windows) click copy all music to itunes music folder, and click keep music organized. Then go into one of the file menus and select consolidate itunes music library. This will copy all of your music files into itunes own secure location. Then you can delete all of your other files to save space, and every time you open a music file itunes will copy and save it in its location and you won't loose music anymore.
At the moment you had that mixed tape and opened it and then deleted the original file or moved it so itunes can't find the file to read off of.