Like many Mac users, I use hot corners to start my password protected screensaver when I’m ready to walk away from my desk. Seemingly out of the blue, I was unable to use hot corners any longer. I tried changing the settings, added another hot corner, etc, and nothing seemed to work. Finally after much googling I found a thread on apple.com with other people having the same problem. Most of the solutions said to reboot which I didn’t really want to do, but eventually someone else found a way to fix the problem:
a) Open up Activity Monitor (/Applications/Utilities/Activity Monitor)
b) In the search filter type “Dock”
c) Select the item in the list who’s Process Name is “Dock”
d) Click on Quit Process.
The Dock and Expose will be restarted and the problem should be solved.
Give that a whirl and hot corners should start working again for you.