Menu disappearing on mobile / small screens

Hello,

My website seems to have an issue with the main menu. If you resize the screen to a smaller size, the menu background will first become transparent, and eventually the menu will completely disappear, to the point that it cannot be seen on mobile at all.

Any ideas why that may be happening? I haven’t changed the site at all in a very long time except for updates, so I’m wondering if a new version of WP caused it, or if it can be easily solved.

Thank you.