I was working on our site yesterday ( www.windhorsethoroughbreds.com ) and the only thing I changed was the background picture for the header. I checked after making the changes and everything showed up correctly. There were 7 updates that I had notifications for and I went ahead and updated them (now I wish I had paid more attention to what specifically was updated) and then I logged out for the day. As far as I knew nothing was wrong as of last night. When I visited the site this morning The main body content on all pages wasn't showing up until you scrolled down below the side bar. I can only assumed something wen't wrong with an update but I'm learning as I go with this and I picked up where someone else left off so it's hard to even know where to begin. I listed the web address above incase you need to look at it to reference what I'm referring to. Thanks!
As @CC said, you are missing a stylesheet. But the big issue is that you're missing a closing
</div> tag above the
</body> tag, and that is throwing the sidebar under the content. I don't know how you're editing pages, but that needs to be fixed. The error may have been there from the beginning.