Try This
I have had a similar problem. I'm guessing that your library is corrupt. Try this. First bring up the menu. You can do this by right-clicking near the top of the Media Player titlebar. Once you have the menu up, go to "tools", then "advanced", then finally "restore media library". A message should come up saying "Continuing will empty your media library and close the Windows Media Player. Do you wish to continue?" Press YES. Windows Media player will close. Open it back up and wait for Windows Media Player to search for music and refresh your library. Make sure that you have Windows Media Player watching your "My Music" folder. The picture below should help...