Andy,
Thank you for your business first of all. Let’s see if we can’t sort this out for you.
For you blog, do you have a page inside the “pages” section that is called blog? Generally when you add a blog to a WordPress site, you create a page. Then you go to the Settings section in your control panel and select Reading. Then you choose a page to be your front page (home page) and another one to be your “posts” page (your blog).
If you set it up that way originally, it’s possible the page that was set to be your blog was deleted. That would make it so have a can’t find page error while you have posts published.
If you didn’t set it up that way, you can go on and do that now. Then go to the navigation builder (Appearance > Menu) and remove the old blog link and add your page in its place.
If you have any questions or trouble, let me know.
Thanks again,
Bill