My music purchase didn't download properly

Depending upon what country that you are in (music can't be re-downloaded in all countries) then try deleting it from the Music app on your phone (swipe/drag from right-to-left across the track) and redownload it via the Purchased tab in the iTunes store app.

Or if using your computer's iTunes delett it from the Music part of your library and redownload it by (assuming that you are using iTunes 12) by clicking on your account name at the top right of the iTunes screen and clicking ‘Purchased’ on the popup.

If you aren't in a country where you can re-download music or if it re-downloads in the same state then try the 'report a problem' page to contact iTunes Support : http://reportaproblem.apple.com

Or you can try contacting iTunes Support via this page : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption