Twenty Twelve Child Theme Menu Toggle Stopped Working

I have a site that uses a child theme of twenty twelve. For some reason the navigation menu toggle has stopped working.

When viewed on a narrow screen the nav menu becomes a toggle, as it should (responsive). Clicking the toggle used to open the nav menu. I’m not sure when it stopped working and I’ve tried to figure out why so I can fix it but no luck so far.

Can anyone offer any help or suggestions so I can get the responsive menu toggle functioning again?

Here is the site:
http://www.telluridemountainclub.org/

Thanks in advance for any help!