OSX Yosemite - Mission Control stopped working

Not sure how long this has been happened, 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 what does it happen, how could I solve it?

Thanks a lot.