Sidebars do not fall to the bottom and display below posts unless
(1) content has been imported into the blog,
(2) the blogger has been copying and pasting content from elsewhere, without using either icon 5 or icon 6 in Row 2 of the visual editor http://en.support.wordpress.com/visual-editor/#pasting-text
(3) or the blogger has copied and pasted invalid HTML into a text widget or left an HTML tag open
The troubleshooting post that houstonweaver linked to will assist you with fixing your blog.