UPDATE 2016-05-16: Spotify doesn’t seem to allow you to drag and drop anymore. Another way I’ve found to fix missing local songs is to create another local files source in Preferences and moving all of the local files to this new folder. Still seems to be hit and miss though.
One of the great things about Spotify is the way it lets you integrate local files into your playlists. These are typically MP3s you have, which are not on Spotify due to licensing restrictions. Once in awhile, things can get out of sync and even though the files exist, the song will appear ghosted in a playlist.
The fix is pretty simple. Just open an Explorer window with the list of your local files and drag the missing song to the Spotify playlist. You will create a new entry for the song, but it will also refresh the old entry. Then just delete the duplicate entry.