Post are not showing up

The reason is that its not showing up

 <?php if ( has_post_thumbnail() ) : ?>
        <a href="https://wordpress.stackexchange.com/questions/206672/<?php the_permalink(); ?>">
            <?php the_post_thumbnail(); ?>
        </a>
 <?php endif; ?>

You are displaying just a link with check that is featured image exists or not

It seems, your post doesn’t have featured image, select any featured image or just remove that check from it, as i can’t see any need of it.

So your code will be :

<?php get_header(); ?>
<div id="main">
    <?php if (have_posts()) : while (have_posts()) : the_post(); ?>
        <div <?php post_class() ?> id="post-<?php the_ID(); ?>">
            <a href="https://wordpress.stackexchange.com/questions/206672/<?php the_permalink(); ?>">
                <?php the_post_thumbnail(); ?>
            </a>
        </div>
    <?php endwhile; ?>
    <?php include (TEMPLATEPATH . '/inc/nav.php' ); ?>
    <?php else : ?>
        <h2>Not Found</h2>
    <?php endif; ?>
</div><!--Main End-->
<?php get_footer(); ?>