When you create a blog in Sitefinity, you must also assign it a Default Page, which is the page that contains the BlogPosts widget for displaying the full Details View of a post.
If you delete this page, move the BlogPosts widget to another page, or modify this relationship in any other way, you must make sure to update the settings for your blog so that it always points to the correct page. Otherwise, Sitefinity will have no way of knowing where your blog posts are located.
This isn't necessarily a problem for your end users, because the BlogPosts ...