OS 10.5 - Screen saver trouble...

For some reason (obviously), my screen saver and screen sleep settings have ceased to have any effect. Neither will come on according to the set schedule. I can induce either by using the hot corners, but they won't activate automatically.

It's worked for months, but spontaneously stopped a few days ago. No new software has been installed, nor have I made any mods to the system.

Any ideas?

Ken