It seems that the comment box disappeared and people can no longer add new comments to my site. It says comments are closed, even though the box to close comments after a certain time is not checked. I don't remember changing anything, but I could have accidentally clicked on something.
At this point, all the plugins are disabled. I tried another theme and the comments box did not re-appear.
Here is an example of a post:
Earlier this morning, the comment box was still there, as I recall. Under "Discussion", "Enable threaded (nested) comments levels deep" and "Comment author must fill out name and e-mail" are checked.