Missing Header

Site URL: https://www.ariellazoelle.com

I haven't made any recent changes to the coding on my website for almost two months, but I did add a new product to my store this week. For some reason, the header with all the navigation links is missing at the top when you visit my website in both desktop and mobile views. When I open the page in Squarespace, this is what I see, where the header is still missing:

But as soon as I hit Edit, the header shows up:

But when I click save, the header disappears again.

What is causing it not to display and how do I fix this? I don't understand how adding a new book to my store could cause this error, and I didn't make any coding changes when I added the new product.

This is making my website literally unusable, so I'd really appreciate help! Thanks. 

Edit:

If you go to https://www.ariellazoelle.com/cart the header appears, but you can't click on anything and none of the drop down menus work or any of the colors change as per my CSS.

But if you go to https://www.ariellazoelle.com/sunnyside the header is still missing:

I don't know if that helps clarify anything, but I'm at a total loss. 

As always, @tuanphan, you have been a TREMENDOUS help. I'm so appreciative of your time and efforts. Thank you for fixing that infuriating problem! ❤️