Hi Terry,
pretty late, but it is just now that I read this thread. Looking at the screenshot, I think this is the same issue that I am sometimes running into. On the taskbar the video window gets shown as a small preview, but it does not appear on the screen, because it is shifted off-screen outside of the visible area. How it gets there? I have no idea, but this is how I get it back:
Hover with the mouse over the Dorico icon in the taskbar, so that the small preview video window appears. Move the mouse to the middle of it and do a right click. From the then appearing context menu choose ‘Move’. The mouse cursor will change to an arrow pointing into all 4 directions (and will move somewhere to the edge of the screen). Now, press one of the cursor keys (up, down, left or right), but only once. The cursor shape will then change back to normal, but if you now move the mouse, the video window’s position will follow the mouse cursor. Position it at a convenient location, make one left-click and the video window will stay.
Hope this helps.