Hi Sarah,
Thanks for writing in! I have checked your site and there is a JS error on the page. Jetpack plugin and Social Warfare plugin is generating this JS error which may have affected the hamburger and the sticky functionality of the header. Please make sure that the theme and your plugins are up to date. After updating, please clear your site caches, plugin caches and regenerate any minified JS/CSS files. I can see that you have Autoptimize plugin installed. Please clear its cache and regenerate it to resolve your issue.
Please let us know how it goes.