OSX Yosemite - Mission Control stopped working
(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.