iTunes Store: Hiding and unhiding purchases - http://support.apple.com/HT201322
Click on this link and read how to 'Redownload music' --> https://support.apple.com/HT211816
https://www.apple.com/legal/internet-services/itunes/us/terms.html
SERVICES AND CONTENT USAGE RULES
"- Purchased Content will generally remain available for you to download, redownload, or otherwise access from Apple. Though it is unlikely, subsequent to your purchase, Content may be removed from the Services (for instance, because the provider removed it) and become unavailable for further download or access from Apple. To ensure your ability to continue enjoying Content, we encourage you to download all purchased Content to a device in your possession and to back it up."