computer freezing downloading El Capitan

I just downloaded El Capitan to my MacBook Air, which I bought in late 2011. The download was successful, but since I started the installation this morning (about six hours ago), nothing has happened. I can't turn off the computer or restart it. What could have gone wrong and what can I do at this point? I'm updating from Mavericks 10.9.5 to El Capitan 10.11.16.