Linux freezes completely except mouse pointer

This is to me a general question, since I saw many questions about Linux freezing (didn't help to apply the answers, including the "Raising Skinny Elephants Is Utterly Boring" keystrokes combinations sequence). It should be general also because it used to happen to me on Ubuntu (the version "LTS" up to date in 2016) and also on Centos 7, which I installed about one year ago, on the same desktop, a 2011 HP Compaq 8000 Elite. Basically everything is frozen, it appears there is no disk activity. Only the mouse pointer moves correctly, but clicks have no effect. Up to now I am obliged to reboot the box. Any suggestions about how to unfreeze the box in these cases and possibly search and suppress the reason?