Try re-opening the editor from the Playmaker menu.
If you don't have a Playmaker menu, make sure you've imported the Gizmos and Playmaker folders from the unitypackage.
This normally happens when you switch to a project that doesn't have the Playmaker editor imported, but the saved editor layout has a Playmaker editor window - unity then can't show the proper editor window. In my experience it's fairly harmless - just close the old windows and open new ones (you can also save/load layouts to quickly restore the layout after re-opening Playmaker).
Hope this helps,
Alex
