If you've recently changed your Thinkific domain, installed a custom domain, or changed your Disqus lesson titles, chances are your course player is no longer able to load the Disqus discussion.
This is because the URL has changed and Disqus doesn't know how to connect anymore. You'll need to remap the URLs in Disqus. They have some instructions for that here: https://help.disqus.com/customer/portal/articles/912757-url-mapper
If that seems like a bit too much, you can do a quick and easy fix by disconnecting and reconnecting Disqus. Keep in mind, however, that this will result in all of your previous Disqus discussion posts being wiped clean.