Using the default (Blue Banner) theme everything was fine until I noticed yesterday the sidebar had moved to the bottom. I switched to the classic theme and it reappeared.
I thought the most recent post might have been a problem but when I marked its status as draft, the problem continued to exist.
I haven't messed with any templates. I experimented with other themse but got the same result.
Then I noticed that Oct. archives looked correct. Novemenber, too. It is just the main page that has moved the sidebar to the bottom.