http://www.hypebot.com/hypebot/2013/09/the-case-of-the-disappearing-tracks-your-music-is-not-safe-on-apples-icloud.html?inf_contact_key=2b5f8b6eac29a8896138ce8fc2dd0973be48293a93d6945333b56ca6112938c3
Seems that you need to ensure you have a copy offline as access is not guaranteed to remain in place. The lesson, do not have your music *only* in the cloud, always have a long-term offline copy somewhere.