Brooks:
I just had the same same thing happen to me, It looked like I lost all the sidebars no matter what theme I changed to, but they were actually way down at the bottom of the web page. I replaced all the WordPress files one by one except for my content. Nothing worked.
It turned out I had a corrupted post somehow. I deleted the last post I had made before everything went haywire, reloaded my homepage in my browser and everything is back to normal. Weird.
