If you have been copying and pasting text from elsewhere without using either icon 5 or icon 6 in Row 2 of the visual editor, please see http://en.support.wordpress.com/visual-editor/#pasting-text
The reason your sidebar has fallen and is displaying below your posts is either due to copying and pasting or it can also be a result of importing text containing invalid HTML.
Typically this is caused by an open HTML tag in one of your posts.
Go to Settings > Writing and select “ ___ WordPress should correct invalidly nested XHTML automatically” and then scroll down and click “Save Changes.”
Now, starting with your latest post, open it in the editor, make one minor change such as adding a space and then deleting it, and then click “Update Post.” Check your blog and see if it is back to normal.