Open Preferences and click the "reset dialog sizes and positions" button on the general tab.
Does this correct the problem?
(If not, quit and restart FileMaker and check again...)
I've had design dialogs--> which can be dragged to a second monitor and outside of the application window, go "missing" when I revert back to a single monitor configuration. This buttons fixes that issue.
PhilModJunk, you are my hero. In 20 seconds you fixed something that had me pulling my hair out. Thank you so much!!!