Well, it told me the problem was in a theme file. A little snooping led me to comments.php. Short version: I overlaid the online file with a version from the April backup, and the comment form came back. Apparently I had done something nonstandard to disable comments, without even leaving my initials as a footprint. Dumb.
The standard comment form does not have a captcha. I think I need a captcha plugin.
