We're having some trouble with Exposé and Spaces. Help?

My dad usually has tons of important files and folders up and running on his Mac’s desktop, so he naturally makes great use of Exposé and Spaces. All of a sudden, when he puts his mouse up into the corner of the screen (his way of triggering Exposé), nothing happens. I went into System preferences and disabled and re-enabled both Spaces and Exposé, but it didn’t work. Any suggestions?

P.S. My Dad would rather not have to restart his computer, since he’ll have to reopen a lot of things.