You have to replace the code of index.php by this code of your theme. Perhaps you will be some changes in index.php due to which error is coming.
Replace the code of index.php:
<?php if (have_posts()) : ?>
<div id="posts">
<?php while (have_posts()) : the_post(); ?>
<div class="post">
<?php if(is_single()) { ?>
<div class="title">
<h2><?php the_time('l jS F Y'); ?></h2>
<span class="date">by <?php the_author(); ?></span>
</div>
<?php } else { ?>
<div class="title">
<h2><a href="<?php the_permalink() ?>" title="<?php the_title() ?>"><?php the_title() ?></a></h2>
<span class="date"><?php the_time('d M Y'); ?></span>
</div>
<?php } ?>
<div class="entry">
<?php the_content('Keep reading…'); ?>
<?php wp_link_pages(); ?>
<?php the_tags('<p>Tags: ',', ','</p>'); ?>
</div>
</div>
<div class="sep">· · · ◊ ◊ ◊ · · ·</div>
<?php comments_template(); ?>
<?php endwhile; ?>
<div class="navigation">
<div class="alignleft"><?php next_posts_link('« Previous Entries') ?></div>
<div class="alignright"><?php previous_posts_link('Next Entries »') ?></div>
</div>
</div>
<div class="months">
<?php if ( !function_exists('dynamic_sidebar') || !dynamic_sidebar() ) { ?>
<div class="title">Category</div>
<?php wp_page_menu(array('depth' => 1)); ?>
<div class="title">
<?php if (($category_link = dd_get_page_link_by_slug('category')) != '#') { ?><a href="<?php echo $category_link; ?>">Category</a><?php } else { ?>Category<?php } ?>
</div>
<ul>
<?php wp_get_category(); ?>
</ul>
<?php } ?>
</div>
<?php else : ?>
<!-- No Posts Found -->
<?php endif; ?>