This error took me a long time to fix by myself because I had no idea what the problem could be and plug-ins happen to play a big part in a lot of what goes wrong in WordPress. I wish had thought about that before fixing the core code of WordPress. I searched Google and found a few solutions and none of them worked. If the reason as to why your comments aren’t display isn’t the same as mine here are a few of the solutions I stumbled upon.
Your comments aren’t enabled in Settings -> Reading
Permalinks are the culprit
Try changing your permalink structures back to default.
You don’t have the proper line of code in single.php
<?php comments_template(); ?> at the bottom of your single.php page, right before <?php endwhile; else: ?>
I tried all of these as well as things I just would rather not discuss because of how time consuming and probably ridiculous they were, but at the time anything that might have made those comments appear again was worth the time. What the problem was for me was I had the Social plug-in, the one that allows you to broadcast to Facebook and Twitter on every Publish of a Post. I hadn’t realized that this plug-in overwrites WordPress’ capability to load it’s own default comments.php template that is located in your template directory. Something had gone wonky with that plug-in, or the comments.php file that was associated with it. I tried to find a way to allow WordPress the ability to load the template I wanted but I simply couldn’t find it so I deleted this plug-in and everything worked as it should. Now it’s time to find a new social broadcasting plug-in, which I’ll more then happily post up once I’ve got one.