Diagnose Theme 404

// Add this code to your theme if you have already run through these basic steps:
// http://www.wpbeginner.com/wp-tutorials/how-to-fix-wordpress-posts-returning-404-error/

add_action('get_footer', 'please_help');

function please_help () {
        global $wp_query;

        $post_count = count($wp_query->posts);
        $is_404 = (is_404() == TRUE ? 'true' : 'false');

        echo "<!--
        Post count: $post_count
        is_404(): $is_404
        -->";
}