If redownloading music is available in your area, I recommend deleting the song from your library and redownloading it through iTunes in the Cloud.
This article will help you determine what content is available for redownloading in your area:
This article will walk you through the actual redownloading of the file:
Downloading past purchases from the App Store, iBookstore, and iTunes Store
I hope this helps! Best wishes and good luck!