- Make sure that the hidden window or window is highlighted. Or at least your mouse is close to the area where you think the application has opened.
- Press Alt+Space, which brings up a popup menu displayed in the viewable desktop.
- Press M, which selects "Move" in the menu.
- Press one of the Arrow keys, no matter which one.
- Move mouse, and the hidden dialog will show up all in a sudden under the mouse cursor.
This is a one time fix. For a more permanent fix, try ShellFolderFix @ Download ShellFolderFix