I hope I'm not stating the obvious, but the 'scroll bar' on the far right side of the screen is scrolled all the way to the bottom of the window. The 'scroll bar' at the very bottom of the screen is jammed to the right and could be pulled to the left a small amount. Move them up and to left respectively. This might bring the 'black box' into the screen limits.
It also appears the narrow scroll bar on the right side of the black box is also bottomed out. Can you drag it up?
Also noted the expansion icon at the bottom right corner of the black box. Did you try to click and hold and drag it down and to the right.
A quick Google seems to indicate Cubase has some glitches with old databases and earlier version numbers and Windows 10. In fact, many other users are indicating both successes and failures using Cubase with Win10. I have an old program I consider so valuable that I maintain an old 32 bit Vista computer just to run it. If Cubase is that important to you, you might consider backing off to Win7 which I understand later versions of Cubase were designed for. I'd suggest studying the user forums. Cubase and Win10 seem to have a lot of threads.
Other desparation measures might include changing the screen resolution, left or right dragging and banging the title bar of the main window to create a half screen image.
Not knowing or using Cubase, I really don't have much else to offer. I know a lot of my older programs stop working when Windows 8 to Windows 10 came out.