The app and all the info have vanished. I was following a forum about getting address book into dropbox and whoosh, it all vanished. I reinstalled the application but my latest phone book entries were not there. I had been trying to sync to the cloud but it never did sync so my latest address book has vanished. Really bad. I am now on MacKeeper so this won't happen again. Thanks for responding! Pendlewitch and Chas.
You still didn't tell us if the app disappeared from the Dock at the bottom of your screen or from the Applications folder itself, which you navigate to in the Finder. It could still be there. Just because something is gone from the Dock doesn't mean it's gone from the computer entirely.
Time Machine is a better option for backing things up than MacKeeper, and it comes as part of OS X already. You should give that a try.