"I accidentally erased the sidebar while working on the HTML. It shows up when viewing individual entries, but not the blog as a whole.
"hbl" suggested I re-add the gadgets on the Layout>Page Elements page. So...
When I go to the Layout>Page Elements page, all of the gadgets are still there. I clicked on each, saved it again, then clicked View Blog, and still no sidebar. If I click on an individual post's title, then the sidebar shows up."
and eventually Aviatrix's suggestion helped:
I took out the post that caused the problem in the first place! (And then replaced it with a duplicate.)
Now my blog is all better. : )
Well, except that it now has a new color scheme. But I don't mind it that much.