My iMac will not restart or shutdown after
iMac (Retina 5K, 27-inch, Late 2014)
If your iMac will not restart, you can try resetting the system management controller. Reset the System Management Controller (SMC) on your Mac - Apple Support
You might also reset the NVRAM: Reset the System Management Controller (SMC) on your Mac - Apple Support
From How to turn on or turn off your Mac - Apple Support::
If you can't shut down your Mac because it isn't responding, you can force it to turn off without shutting down. Hold down the power button for 5 seconds, until your Mac turns off. Use this method only when you can't shut down. You'll lose any unsaved changes to documents that you have open.
Hello !
Hold power button, for 1.5 seconds while your Mac is awake to display a dialog asking if you want to restart, sleep, or shut down. If you don't want to wait 1.5 seconds, press Control–Power button or Control–Media Eject 
i cannot restart my iMac for update
