I've tested on Edge, IE 11, latest Firefox and latest Chrome and the menu is always there until you begin to scroll down.
Looks like the menu container is there but the font is maybe the wrong colour for you. Do CTRL+F5 (CMD+R on a Mac) on the homepage. That'll make sure it gets the latest files from the site and doesn't used locally cached versions.
Although not quite the issue here, I must say, that's a bizarre design choice to have the search stick to the top but not the menu with it.