Everything had been working fine. I made no changes to code, nor added any new plugins, and then:
My sidebar disappeared, and the footer started to show at top of webpage.
After isolating the plug-ins we found that WP Super Cache was responsible.
I also got an error email generated from webserver that read:
"Error. Supercache could not write to wp-content/cache/supercache/www.alterpolitics.com/images/AlterPoliticsLogo.gif/581491434d0d1355a629e0.07342954.tmp"