How to make message show for no posts in this WP_QUERY statement?

I am trying to use this WP_QUERY code to display a list of custom posts. The code below works fine, but I am confused how or where to add information if thee are no posts. For example add an if/then or while/if/else statement? I tried different mthods but they all broke the PHP, so any idea how to add something at the bottom that displays "Sorry, no posts match your query." message?

//Define your custom post type name in the arguments
$args = array('post_type' => 'classified_ad_data');
//Define the loop based on arguments
$loop = new WP_Query( $args );
//Display the contents
while ( $loop->have_posts() ) : $loop->the_post();
<h1 class="entry-title"><?php the_title(); ?></h1>
<div class="entry-content">
<?php the_content(); ?>
<?php endwhile;?>