Screensaver won't go away

In the past two days, I've twice had my screensaver get "stuck." The computer doesn't go into sleep mode (as it usually does, with a black screen), and clicking the mouse or hitting a key on the keyboard doesn't dismiss the screensaver.

Both times, with a lot of futzing around, I've finally been able to get it to go away, but I never can tell what did it. The first time it went away shortly after I unplugged and plugged back in my keyboard, and turned the bluetooth Mighty Mouse off and back on.

My Mighty Mouse periodically just spontaneously loses connection, and I can't tell, when the screensaver is hung, whether that has happened or not, but turning it off and back on always restores the connection (after it syncs back up), and I can't understand at all why my keyboard no longer dismisses the screensaver, which it has always done before this started.

I can't find anything anywhere about anyone else having this problem. Does anybody know what might be causing it?