Banner above/below all post titles

I’m trying to place a (728×90) banner above or below the titles of all my posts.

I read multiple guides and forum posts about it, they all basically state:

Place banner-code below this in single.php:
<?php while ( have_posts() ) : the_post(); ?>

When I look at twentyfourteen single.php file this line looks like the one I need:

while ( have_posts() ) : the_post();

But whatever I place anywhere in single.php file, the result is a blank page after refresh. Apparently, anything I place inside single.php seems to break any posts.

I have found multiple similar questions out there, but not got resolved unfortunately…

I hope someone knows the answer: what file to edit and where to place the banner code to have a banner above or below the post titles?