posts not showing; categories show instead

Hello!

I had my site running well for months until I realized the index page with my thumbnails (links to posts) seem to no longer link to the posts properly. when I click on a thumbnail, it instead shows me the same page, only with fewer thumbnails that seem connected by a category. Very strange!

Here is the code for my index.php and single.php below

index.php:

<?php get_header(); ?>

<div class="home fix">
  <div class="main">
    <div class="fix">

<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
<?php
   if (is_home()) {
      query_posts("cat=4");
   }
?>

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

<!-- thumbnail wrapper -->
<div class="thumb main">

<!-- 235150image-covers -->
<?php $image = get_post_meta($post->ID, 'thumbnail', true); ?>
<a href="<?php the_permalink() ?>" title="<?php the_title(); ?>"><img src="<?php echo $image; ?>" alt="" /></a>
<!-- 235150image end -->

<!-- thumbanil title -->
<div class="thumb-title">
<h2><a href="<?php the_permalink() ?>" title="<?php the_title(); ?>"><?php the_title() ?>
 <?php comments_number('[+]', '[1]', '[%]' );?></a></h2>
</div>
<!-- thumbanil title end -->

  			</div>
  			</div>
  		  <?php endwhile; ?>

  		  <?php else : ?>
  		  <div class="post single">
    			<h2>No matching results</h2>
    			<div class="entry">
    				<p>You seem to have found a mis-linked page or search query with no associated or related results.</p>
    			</div>
    		</div>
  		<?php endif; ?>

  	</div>
  </div>

</div>

<?php get_footer(); ?>

And here is the code for single.php:

<?php get_header(); ?>

<div class="home fix">
  <div class="main">
    <div class="fix">
       <?php if (have_posts()) : while (have_posts()) : the_post(); ?>
  			<div class="post single fix" id="post-<?php the_ID(); ?>">
  			  <h1><a href="<?php the_permalink() ?>" title="<?php the_title(); ?>"><?php the_title() ?></a></h1>
  			  <div class="postMeta">
              <span class="date"><?php the_time('M jS, Y') ?></span>
			  <span class="cats"><?php the_category(', ') ?></span>
              <span class="edit"><?php edit_post_link('Edit this entry.'); ?></span>

              </div>

<div class="entry">
<span class="meta-data">
<?php

  $mykey_values = get_post_custom_values('Brief');
  foreach ( $mykey_values as $key => $value ) {
    echo "<strong>Brief:</strong> $value<br />";
  }

?>
<?php

  $mykey_values = get_post_custom_values('Media');
  foreach ( $mykey_values as $key => $value ) {
    echo "<strong>Media:</strong> $value<br />";}

?></span>

		<?php the_content('<p>Continue reading &raquo;</p>'); ?>
	<br />

  		          </div>

  			</div>

<div class="prevnext">
           <div class="left"><?php previous_post_link('%link', '&laquo; %title', FALSE); ?></div>
        <div class="right"><?php next_post_link('%link', '%title &raquo;', FALSE); ?></div></div>

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

  	</div>
  </div>

</div>
<div class="commentpage">

<?php comments_template(); ?>
</div>

<?php get_footer(); ?>

Any help would be greatly appreciated!