With a quick glance at the code, it looks like you are using the global $wp_query, but you haven’t declared it. Before you can use this you would need to declare: global $wp_query;
I’m not sure why you even have this bit in your, code though. Your logic looks like it’s trying to check if there is only one post, then show the excerpt if so, then you go and show the excerpt on else anyway.
Can you give a more detailed explanation of what you are trying to accomplish?
// Get the most recent post
$the_query = new WP_Query( 'posts_per_page=1' );
// Pull the excerpt
while ( $the_query->have_posts() ) : $the_query->the_post();
// Reset Post Data