Could someone please explain why when I submit an empty search form I get sent to a page with no navigation menu? The navigation menu is there when a search term that has no matches is submitted and the error message is the exact same: "Sorry, but nothing matched your search criteria. Please try again with some different keywords."

looking at the seach.php file there's only one else clause that handles the no result found code so why isn't functioning the same? I haven't modified the search from the original theme

<?php
get_header(); ?>

        <div id="container">
            <div id="content" role="main">

<?php if ( have_posts() ) : ?>
                <h1 class="page-title"><?php printf( __( 'Search Results for: %s', 'twentyten' ), '<span>' . get_search_query() . '</span>' ); ?></h1>
                <?php
                /* Run the loop for the search to output the results.
                 * If you want to overload this in a child theme then include a file
                 * called loop-search.php and that will be used instead.
                 */
                 get_template_part( 'loop', 'search' );
                ?>
<?php else : ?>
                <div id="post-0" class="post no-results not-found">
                    <h2 class="entry-title"><?php _e( 'Nothing Found', 'twentyten' ); ?></h2>
                    <div class="entry-content">
                        <p><?php printf( 'Sorry, but nothing matched your search criteria. Please try again with some different keywords.', 'twentyten' ); ?></p>
                        <!---<?php get_search_form(); ?>-->
                    </div><!-- .entry-content -->
                </div><!-- #post-0 -->
<?php endif; ?>
            </div><!-- #content -->
        </div><!-- #container -->

<?php get_sidebar(); ?>
<?php get_footer(); ?>

Sounds like something breaks when you submit an empty form. It could be as simple as an unclosed tag in that situation. I'd search for issue in that specific case.

I appreciate your response however I have checked and haven't noted anything missing. Also it looks to me that an empty search form and a search form with no matches is processed by the same else clause. Have you had experience with Wordpress? specifically Twentyten theme?

Also it looks to me that an empty search form and a search form with no matches is processed by the same else clause.

Make sure.

Have you had experience with Wordpress? specifically Twentyten theme?

No. Just seen a lot of bad coding...