Same problem on my mobile site of dare2go.com The menu flashes briefly onto the screen and then is gone again. It doesn’t matter if you keep your finger on the “hamburger” icon or not.
Before the latest theme update my mobile menu, including sub-menus, was working just fine!
[ps: I have reverted my site to version 3.4.5, so it’s working – but NOT with the latest theme version…]