I know it's been a year, but the same happened to my mum today. We fixed it by changing the screen resolution. It was showing (you can tell by the main scanning window going out of focus) but outside the visible screen. So I changed the resolution to the highest possible and there I discovered it. You can also try (when you click Preview and get the focus) to press Alt+Space and use arrows to move it to a visible screen location.
P.S.: Should have waited until tomorrow when it'll be exactly a year!