Well, I don't know what you've done, and I'm no computer expert, so take all of this for what it's worth!

Try playing the song with the Limewire player, it's located at the bottom of the screen towards the right. It may be corrupted.
Check the song format, it may not be compatible with WMP.
Redownload the song from a different source.
If all that fails, try downloading Winamp. It's a bit more reliable than WMP. You can download a basic version for free at Winamp Media Player - MP3, Multimedia, and Music Player.

I hope some of this helps!