The comment title is located under the comment form on the top of all comments with comment filtering and ordering buttons:

However, you can add it back very easy, putting this hook code in the WordPress active theme functions.php file (more information how to add insert custom code):

add_action('wpdiscuz_comment_form_before', function() {
   echo '<h3 style="max-width: 750px;margin: 0px auto;">' . __('Leave a Reply', 'wpdiscuz') . '</h3>';
});

Related topics: 

• https://wpdiscuz.com/community/troubleshooting/css-styling-help/
• https://wpdiscuz.com/community/troubleshooting/how-do-add-the-header-or-hide-the-form/