Navigation Menu Disappears when desktop browser is resized or on iPhone

Hello, I love Catch Box (free). It is a really nice, elegant, and simple theme. So thanks! I'm having trouble with the Main Navigation Menu disappearing when the browser is resized and also when on an iPhone. I don't have a live site to show you, but I can assure you that is what is happening. How can I get the menu to be responsive with the rest of the site, but still show when the browser size is small or when on an iPhone? Thanks again! - Chris