Okay, it's back:
https://www.dcurbanmom.com/jforum/posts/list/1308872.page
A record for a topic in the database contains the id of the first post and the id of the last post. The id of the last post in that record is for a post that doesn't exist. I don't know how this happened, but apparently some glitch someplace. I corrected the last post id and now the thread is viewable again.