Sidebar misplaced below page

I have been running my blog for years without problems and now all of a sudden on the main page the sidebar is stuck underneath the first page of posts. It’s fine on the individual posts. I am using Gravel theme.

The sidebar area should look like this:

http://www.chinabluepublishing.com/bsblog/2014/07/review-the-raven-boys/

The main page looks like this (sidebar missing):

http://www.chinabluepublishing.com/bsblog/

Except for regular WordPress and theme updates, I haven’t knowingly altered anything. I’ve tried to contact the theme’s creator with no response. Any ideas? I’d appreciate any help. Thanks!