Go back to the main Lessons page.
If you’re seeing this, you clicked on the menu button on the main Lessons page of this site, or one of the subsidiary pages. Chances are you have JavaScript turned off; since you can’t see the menu on the page as it is designed to appear, this page is probably your best bet for accessing the Lessons menu.
If you can access the fancy menu on the main page, and you prefer it to this one, you can go back to the main Lessons page.
It’s debatable whether relying on a JavaScript-driven menu is best practice, but I like the menu so much I just went for it anyway.