You open zookeeper, you see it is open in the tool bar but you can't see the window. It is most likely on another output under the Hippotizer Engine output.
Here are a couple of ways to get it back,
1, Hold Win key + Shift and use left and right arrow keys to move the window to different outputs. 2, Press Alt + Spacebar, press M, now use the arrow keys to move the window up down left right in pixels.