Here's an old post that has 218 comments, but none are displayed:
Disabling Google+ comments fixes this issue, but the comments powered by Google+ are missing. This means that the old comments aren't lost, but there's a bug in the Google+ commenting system.
I checked the Blogger help forum before posting this and I was surprised to see some top contributors claiming that "all the non-Google+ comments disappear when you switch to Google+ comments". I'll link to the official Blogger blog: "older comments will continue to appear in the new widget".
Update (a few hours later): Google fixed the bug, but there's another one. I can't post a new comment and clicking "reply" doesn't have any visible effect. I've seen similar reports in the Blogger help forum.
Update 2: Google acknowledged the second issue and promised to fix it.
Update 3: Apparently, blog owners were the only ones that couldn't post, but this was fixed. Thanks, Yonatan Zunger.
{ Thanks, Kristian. }