This happens often that keyboard shortcuts stop working like volume control, screenshot, etc., and it start working again when I reboot system. this issue started from last month only, never had this issue before. When I press "Window" button it pop-up with list of shortcuts as usual but shortcuts doesn't work.
Again as I said all starts working after reboot. only pain is that you're working on something and you want to take screenshot and doesn't work and you've to reboot.