This is my site:
Today, my top-level menu items stopped working.
If I change my permalinks back to the default then those menu items work again but it screws the blog. When I do that, none of my back-links work.
If I keep the Permalinks as month and name, the back-links work and the top level menu items don't. I can navigate to child-pages like here: https://neighbourgood.co.uk/about/in-loving-memory/
The most radical thing that happened on the back end prior to these changes was an up-date to WordPress SEO by Yoast.
I haven't a clue what to to do next but I have been through and disabled all plugins including the SEO one.
Hope someone can help?