Everyone once in a while things go awry for me on OS X.
Symptoms often include hot corners suddenly no longer working, and spaces not switching when I switch applications.
Turns out the restarting your dock can fix this in a jiffy.
I’m a command line kind of guy, so open up terminal and type this in to restart the dock:
killall -KILL Dock
It looks pretty scary (“Kill! KILL!”), but all it’s doing is telling OS X to kill the Dock process for you, which it will auto-restart. Your dock will disappear for a moment and then come back in working order.