Rebuilding single.php file

Hi guys! I’m in a bit of a bind here. I’m working on a pre-existing WP site for my client. The people who originally set it up really made a mess of things.

Whenever a new post is made on the site, instead of showing the post’s content it will show the same “filler text” every time. I checked the single.php file for the site’s custom theme and that filler text is written into it. I can’t really find a place where it will grab the actual post’s content. I’m trying really hard to keep the existing custom theme because my client likes the look of it, but I don’t know exactly how to rebuild the template so that it will show the post.

Any help would be greatly appreciated!