If choosing it from the Page Layout Tool and menu, it may somehow have been moved off the screen. Changing your screen resolution (temporarily) to make everything smaller may show you enough screen real estate to retrieve the window.

Alternatively, IIRC trashing (or better yet temporarily moving and renaming) your preferences file (is it called Finale.ini?) and then rebooting and restarting Finale should reset your defaults and bring the window back to center. Be careful with this. You may want to wait to see if Peter or one of the Mikes jump in to confirm my recollection.

Already tried both methods and they didn't work

Got it!!! After juggling three hours with the extend/duplicate displays, alternating which one was main display, it suddenly appeared. Cannot say I have a method, though.

Thanks to all.