Modify Search Query if original Query gave no results

You should create a custom query inside if.

if ( $total_results == 0 ) {
    // Create a new query with the terms 'Find Us'

$query = new Wp_query('s=Find Us');
   if ($query->have_posts()):
                while ($query->have_posts()):$query->the_post();
                //whatever you want to do with the result
                endwhile;
            endif;
       wp_reset_postdata();
}

You can refer Creating a search page and Wp_query class referrence for more detail.