I have this problem about 40% of the time. I'm going to try and put the task manager always on top, as suggested. Another work around I found is that my taskbar still appears, so I click on the program switching icon (in Windows 10 - looks like a window with two windows on either side, smaller like they need to be selected to get bigger - sorry, bad description) and right click to put the task manager on a "new desktop", which I can then use to kill Civ. That said, the task manager always on top sounds way easier.