Install Mountain Lion now that Mavericks has been released
It seems that you bought Mountain Lion but now it has disappeared.

You can unhide purchases (some get hidden or you hid them, does not matter really) by opening Mac App Store.

Then go to: Store->View My Account->Enter AppleID and Password

There should be a "Manage"-Button for Hidden Purchases



Find OS X Mountain Lion and click "Unhide"

Afterwards Mountain Lion should show up in your Purchases Tab and you can reinstall.