In the Mac OS, when you operate dual monitors the computer sees both monitors as two parts of one giant desktop. When you go back to one monitor, the computer is still using the setting for the larger desktop resolution so documents (or icons or anything) that were on the second monitor are still there, just too far off to the side to be visible on your remaining monitor.
In Final Draft, go to Window > Zoom Window. This should bring any open document back to the upper left corner of your screen.
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