For future reference there is a key combination that lets you grab any offscreen dialog and move it into view.
So with the focus on Preferences (in other words you have tried to open it and the machine is going bing) press alt and spacebar together, then tap m once (where m is move in your language, would be d in french for example) then tap any arrow key and without clicking move the mouse around until the Dialog appears on screen.
Once you have Preferences you can use the Reset Workspace button which should bring all dialogs back to the screen.
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