So I decided to make a few changes on my blog, like a new header. Which worked perfectly. Then I decided to get rid of Adsense. Blogger Help told me how to do so -- look up a certain section of code, delete, and republish.
Ah, in my younger days, I was more careful. Before making any changes, I would copy the entire coding into a Word document, save it, and then preview changes in my template before saving them.
I followed Google's instructions precisely, and now my sidebar is way over there. I can't figure out why.