Q: Black screen slow and glitchy and no dock

Greetings Bojams,

Thanks for using the Apple Support Communities!

I understand that after updating your Mac to the most recent version of El Capitan, your display is black and you do not see the dock. 

To troubleshoot this situation, I recommend holding the power button to shut down your Mac as described in the following article. 

If your Mac won’t shut down, press and hold the power button for 5 seconds to force a shutdown.

OS X El Capitan: Log out, sleep, restart, and shut down

Once the Mac has been shut down, try starting your Mac in safe mode. The next article describes this process. 

Try safe mode if your Mac doesn't finish starting up - Apple Support

Have a great day!