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.
In Elementary 5 (Hera) this can happen when gala (the window manager) crashes. You can recover from a situation like this without rebooting by running:
setsid ibus-daemon &
ibus restart
setsid gala --replace &
The first command, ibus-daemon starts the ibus daemon, which likely is needed because it's no longer running - you can verify that before running the command by runnning ps -e | grep ibus-daemon to see if it exists.
The second command ibus restart hooks into the now running daemon.
The third command restarts the gala window manager because it seems when ibus is started before it most keyboard shortcuts don't work, so we "reload" it.
ibusrelated issue.