404 not showing when it should

My 404 page is not showing up when i type in malformed urls. All I get is a page with the header and footer.

I’m using a customised theme but can anybody tell me which files I should have an else statement that pulls the 404 template?

I have the else statement in my index file but I think I might’ve customised the theme to the point of breaking it 🙁

Here is my index.php code:

<?php get_header(); ?>

<div id="pagee">

	<div class="wrap">

		<h2>Archive</h2>    	

    </div> <!-- pagee wrap -->

</div><!-- pagee -->

<div id="main">

	<div class="wrap">        	

       <?php get_sidebar(); ?>  

    	 <div id="content">

          <?php if (have_posts()) : ?>     

        		<?php while (have_posts()) : the_post(); ?>    

			<div class="post" id="post-<?php the_ID(); ?>">

        	<h3><span><?php the_time('F j, Y'); ?></span><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title(); ?>"><?php the_title(); ?></a></h3>                	

            <?php the_content('Read the rest of this entry &raquo;'); ?>           

        	<p><a href="<?php the_permalink() ?>" >Read more</a></p>

        	</div> <!-- post -->

        	 <?php endwhile; ?>     

        <?php if(function_exists('wp_pagenavi')) { wp_pagenavi(); } ?>  

        	 <?php endif; ?>
        			<?php else : ?>

			<h3>Not Found</h3>

			<p>Sorry, but you are looking for something that isn't here.</p>

	<?php endif; ?>
        </div><!-- content -->

    </div><!-- main wrap -->

</div> <!-- main -->

<?php get_footer(); ?>

Can anybody help?