Menu mobile version disappeared

I just saw today that the menu button of the mobile version of my website travellousworld.com. It’s supposed to look like three lines on the top left of the website (I’m using an Android phone). I use the Christine WordPress theme by themesart.com. I didn’t install any new plugins, nor did I change anything of the lay-out. The only thing I did was updating my WordPress version to the newest version. Could this cause the issue? Note, I’m absolutely not a technical person, so any easy-to-follow help is much appreciated, thanks!