I upgraded to Mountain Lion last night, now I've just gone to look up an address book entry and it's completely empty. I can't see any entries in my address book, my mail doesn't recognise names and my Exchange account can not be searched. I've verified that my contacts are available in iCloud. Opening address book just shows a title of "Group Label" at the top left side, no entries available ---------- Turns out my contacts disappeared after adding my twitter account. Removing my twitter account made my contacts reappear.