According to the doc:
The core of the WordPress data is the posts. It is stored in the wp_posts table. Also Pages and navigation menu items are stored in this table.
And if you look in the wp_posts table of a site with a menu you can see records with no content but with a post_status of nav_menu_item. This seems to be how the menu is built.
But in my wp_posts table of the site with no menu, I don't even have these records with nav_menu_item so I don't think I can reconstruct this or at least it seems like it would be too difficult.
Is there a way to delete and rebuild this table? I only have a few pages and I have that content. I don't have any blog posts.
Or do I need to do a complete reinstall of WP?
And yes, next time I'll start backing up the db before I start changing things.