Power
Modern PCs have power switches controlled by software, rather than the big snap toggles my old PC-AT used to have. This means that even when they are "off," there is still juice flowing in there, and still circuits that are hot.
I have solved a number of "hung computer" problems by simply turning the computer off and actually pulling the plug out of the back for a few minutes until it's Really Most Sincerely Dead. I assume pulling the CMOS battery for a while does the same thing.