WordPress/wp-content/themes/twentyfourteen/404.php

<?php
/**
 * The template for displaying 404 pages (Not Found)
 *
 * @package WordPress
 * @subpackage Twenty_Fourteen
 * @since Twenty Fourteen 1.0
 */

get_header(); ?>

<div id="main-content" class="main-content">
	<div id="primary" class="content-area">
		<div id="content" class="site-content" role="main">

			<header class="page-header">
				<h1 class="page-title"><?php _e( "Oops! That page can't be found.", 'twentyfourteen' ); ?></h1>
			</header>

			<div class="page-content">
				<p><?php _e( 'It looks like nothing was found at this location. Maybe try a search?', 'twentyfourteen' ); ?></p>

				<?php get_search_form(); ?>
			</div><!-- .page-content -->

		</div><!-- #content -->
	</div><!-- #primary -->
	<?php get_sidebar( 'content' ); ?>
</div><!-- #main-content -->

<?php
get_sidebar();
get_footer();