How to find lost Mission Control?

Open System Preferences>Keyboard>Shortcuts select Mission Control in the left column and review which shortcuts are activated and what key board shortcuts are assigned to them. Check the box to make the shortcut active, to change the keyboard shortcut assigned to the action click on the key combination on the right righthand side and enter your new combination.

Did you try control - up arrow?

If you are trying to access Mission Control from the Desktop, try control - up arrow. On my older keyboard, the F3 key will also access it.

Control up arrow does nothing.

This seemed to start after I did the latest update for High Sierra on July 19th

Go to your Applications folder and drag it back down to your Dock.

Try control - down arrow.

In my case it's not that the pathway to Mission Control is not there, it's that the function has changed. It used to spread out the open windows on the desktop so that I could see what is there, now it hides all the windows which is much less useful.

That brings the window that I am working in into the center but doesn't show my other windows in that desktop.