Songs that I purchased on iTunes (via iPhone) are now disappearing

For some reason certain songs files that I purchased on iTunes, via my iPhone, are disappearing off of my Mac. Take Stacy's Mom. The folders "Fountains of Wayne" and the album folder are still there. But the music file has disappeared off of my app. Any thoughts how to get them back? Also, how do I resend my Mac so it has all the music on the iPhone?