I sometimes experience whole Ubuntu freezes (from 14.04 to 16.04, I upgrade by a clean installation), just mouse can move (that's why this question is different from others). I think this is caused by one of the cores stop working when 100% load by unresponsive Desktop Environment. There isn't any magic key combination like Ctrl+Alt+Del or Ctrl+Shift+Esc known from Windows. The only thing I could do is hold power button for 5-10 seconds and hard shut down. Is there any other recommended solution?
Could I e.g. with some keyword combination reset CPU or kill all additional services or reset Unity or force soft reboot or I don't know what else could help?