This has been happening with Cubase 9 and up. At least since I work on 4k displays. I randomly loose the top toolbar/header for some reason and I simply cannot get it back without closing and restarting Cubase.
Weird thing is, when I close the session, is seems like it also closes Cubase. As if when the the header disappears, Cubase is closed but the session remains open. Very odd behavior anyway.
Has anyone ever had this issue?
Don’t know if this helps, but…
I have a 4k display (and a 1080 one), and I don’t lose the toolbar, but I have a related problem: the menus become tiny when I mouseover, and don’t line up,e.g. hovering over “Studio” opens a tiny “VST Cloud” menu. I have to jiggle back and forth for the right menu to open.
Been like this since at least 8.5
I don’t have this issue and I too mix match resolutions. I have a 4k, a 1080p tv and a 1920x1200 monitor. One issue I used to have with the 4k was that the mouse pointers and tools would be so tinny like they were stuck on a smaller resolution. That went when away with Cubase 10 but not the disappearing top menus bar.
I seem to remember reading about this similar issue. I believe the answer was to not have the main CB page set to “Maximized”. So try “Restore Down” and pull the page to the size you want.
Regards 
Once the menu bar is gone, I cannot restore anything. All that’s left are the separate windows like the project window, the mixer, editors etc…
I’m using Display Fusion Pro to split and my 55’’ 4k monitor into a bottom 3840x1440 and two 1080x720 virtual monitors. Maybe for some reason that confuses Cubase at some point. Maybe it has nothing to do with this.
So It would be good to know if anyone else is having this issue.
Thanks,