I started using Apple iTunes11, and found the feature where you could play music directly from the cloud very handy. But suddenly, all of my iCloud music disappeared. I thought I would be able to bring them back by selecting Menu > Show Music in the Cloud, but it was grayed out. Fortunately, there is a setting that fixes this. Just follow these steps.
1. Select the menu in the upper-left corner of the window, then select Preferences.
2. Select the Store tab.
3. Check the Show iTunes in the Cloud purchases.