I must have made a little typo in my label... because after I saved it, not only were the topic numbers gone, but also all the blocks on the right side of the page. O.o
I looked very carefully all over the page with editing turned on, and I couldn't find the command icons for the empty label. (I'd already successfully added an empty label with CSS for getting rid of the Topic outline header, so I knew what I was looking for.)
What I finally ended up doing to fix it was back up the course--uncheck the box beside that particular label--and restore to the same course, deleting all the data first. A drastic solution to get rid of one wonky label and retrieve all my missing blocks, but it worked--and the numbers were back as well.
Thought I'd share just in case anyone else ever has a similar crisis and can't find their invisible label anywhere. The course I was working on was a help page, not an active course with grade data. If it had been, I would definitely have restored to a new course instead of overwriting the same course.