Sure thing. The code button on the edit form here inserts a
` before and after your text. That way the HTML parser formats that section via CSS and does not mung your code. It makes it more readable.
(Jan sips more coffee.)
You started with a copy of wp-content/themes/default/single.php
and inserted your video code, right? But that seemed (per your two posts) to display the video but not the post itself.
That meant you broke the loop that displays posts. That loop in single.php starts with
<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
displays the post content via
<?php the_content('<p class="serif">Read the rest of this entry »</p>'); ?>
and ends with
<?php endwhile; else: ?>
What I am suggesting is that you start editing a copy of single.php and take baby steps. Insert a place holder first where your video code should be. Once that’s working, insert your real code between the markers.
That way, if something breaks, you only need to debug the code between the markers.
If you still have problems, then you can paste the code into https://wordpress.pastebin.com/ and post the link here. That’ll make looking at your actual code much easier.