Are you using the default menu, or did you select a menu defined in the Appearance : Menus section? That is where “Custom Menus” are defined.
If the menu you select there happens to be empty, or now undefined, no menu will show. That is the only think I can think of that would lead to no menu being generated – some faulty definition or selection in the custom menu admin page. Either that, or the Weaver option for hiding the menu has been selected. No other way to have no Menu HTML generated – other than perhaps a defective plugin.