Synchronize or refesh
If you downloaded your music for a service such as Rhapsody, \Napster, etc, and let your subscriptino lapse, it will give you this message.
Also, if you ripped songs from a CD, and selected the option to add media usage rights (it is unchecked by default), it added an unwanted license to the player that will give you problems such as these. If this is the case, right-click the song, select Properties, and look at the Media Usge Rights/License/or other similar tab and deselect the license (if it will let you). If it doesn't allow you to deselect the license, you will have to redownload the songs. Songs in mp3 format are the best, and you will not give you such headaches.
-Tha Mp3 Doctor