OSX Yosemite - Mission Control stopped working Not sure how long this has been happening, but I just noticed it AFTER Yosemite update. 

Mission control just does not work: using hot corners, keyboard shortcut, or even double clicking Mission Control.app, there is no way to make my Mac go to Mission Control/Exposé. By the way, I have hot corners for showing desktop and app windows exposé, neither of which work. Hot corners for dashboard and Notification center work as usual.

Any ideas why does it happen, how could I solve it?

Thanks a lot.
(This irritating bug seems to be biting me more often after updating to 10.10.4)

When this happens and I don't want to have to reboot, I only restart the "Dock" process and the problem is gone.  There's probably some GUI way to do it, but I just drop into Terminal and do:

killall Dock


But before that, run killall -s Dock to see what would be killed, to make sure it's only going to directly kill the one "Dock" process. 

Once you run killall Dock, Dock will restart in a few seconds and then Mission Control is working again. You can verify that the restart happened by checking the Process ID of "Dock" after the restart.