menu disappeared

Hi, I have made a child theme from twentytwelve and created a special responsive front page (thanks the new grid helper in Dreamweaver). It all worked just fine, but then I updated twentytwelve, and the menu styles disappeared. Now it’s only a list, and no button forms when I shrink the design down to mobile size, as it did before.
Yes, I have all my own styles in the child theme, I used the default twenty twelve for the menu.
I must have missed something, but what?
The only thing in my child functions is registering of a couple of extra sidebars. Nothing else.