I purchased a song and when i went to listen to it only part of the song played, How do u reinstall a song?

I had downloaded all of my songs from the cloud. I had lost my playlist. I went to listen to a song on the list and it only played a part of the song and then went to the next song. Ive tried listening to the song on my nano and it to only has part of the song. How do I redownload a song I purchased? It isnt saved right in the cloud either