Comments section disppearing completely

In my blog, I had just one post, and the comments section disappeared completely. This happened yesterday. I created a new post then, and the comments section were visible on this new post. After publishing this, I made several changes to the extract(summary) section of the post.

This morning when I checked, I found the comments section is no longer visible on the new post as well !

Also, there were a large number of spam registrations on my blog. I deleted all the spam users though, and now have included a captcha.

Please advise why the comments section has disppeared? It has not got to do with the plugins, because if I create a new post, comments section appears on that post!