To Fix this problem:
It sounds like you have a corrupted Database, so you need to delete it and rebuild it, by re-adding your media to your database.
NOTE: These steps will delete your media list, so you should make a backup of any of these files before you do this, especially if you have purchased music via one of the online music shops. I would recommend just cutting all your music and placing it in a
different folder through a different directory.
I.E If you music is located in my music, then cut it and paste it in a temporary folder in My Documents.
First: close your Media Player.
Second: go to the start menu.
Third: go to your control Panel.
Fourth: go to Administrative Tools.
Fifth: go to Services.
Sixth: Scroll down to the Ws, and right click on everything that says Windows Media Player and click stop. Do it for each one. Most of you will only have one; Windows Media Player Network Sharing Service.
Seventh: Click Start Menu
Eighth: Copy and Paste:
%USERPROFILE%\Local Settings\Application Data\Microsoft\Media Player\
into the Search Programs and Files search bar, and press enter.
Ninth: You will see a bunch of files. You can just delete them, but I would back them up to a different folder first.
I.E Cut it and paste it into a Temprary Folder somewhere else on your drive(My Documents works fine.)
Restart Windows Media Player.
Add all of your music files back to the Library Folders(Music), and it should automatically detect them all.
After it does this you can clean up:
Delete the files you backed up, that were found in the
%USERPROFILE%\Local Settings\Application Data\Microsoft\Media Player\
folder.
Afterwords, return to Services, and renable the services that begin with Windows Media Player!
How to get there:
(go to the start menu.
go to your control Panel.
go to Administrative Tools.
go to Services.)
Your problem should be fixed now!
*Edit
Didn't see the last post that had an explanation on how to do this as well.
Realized it AFTER I typed out the entire way to fix the problem, from my memory.
Fail.