SONG WILL NOT LAUNCH AFTER PURCHASING

If this is just one or a couple of tracks purchased from the iTunes Store that exhibit this problem, delete the tracks from the iTunes library, allowing iTunes to move the track files to the Trash. Then download the tracks again from the iTunes Store via this procedure:

http://support.apple.com/kb/ht2519

That fixes the problem for many people. If problems with the tracks persist, go here:

iTunes Support

and follow the instructions to report the issue to the iTunes Store.

Regards.

The instructions to delete it don't help if the song never showed up in your iTunes library.

Which is a completely different problem from the original poster's question asked back in July of last year.

Regards.