If the Lemur Editor window is not visible, or unresponsive to mouse clicks, it is possible you have the Settings window open somewhere in your Desktop. This problem can occur in multi-monitor setups. The easiest way to resolve this issue is by deleting the Lemur Editor preferences file.
In your Finder, navigate to your user Library:
On the latest versions of OS X, the user Library folder is hidden. To access it, click on Go in the Finder's menubar while holding the alt key. The Library will appear in the drop-down-menu.
Find and delete the following files:
Now you can restart Lemur Editor and the window positions should be reset.
Sometimes an application can get lost between multiple monitors. If you have any applications such as SwitchResX running, try to reset the resolution setup.