This is most often caused by an open HTML tag in one of your posts. It can also be caused by copying and pasting from other web sources or other computer applications that do not have, or produce clean code.
Go to settings > writing and select “WordPress should correct invalidly nested XHTML automatically” and then click “save changes.”
Now, starting with your latest post, open it in the editor, switch from the visual tab to the HTML tab (waiting for the page to refresh) and then switch back to the visual tag (again waiting for it to refresh), and then click “update post.” Check your blog and if it is back to normal, you are golden. If not, open the next post and do the same thing till it goes back to normal.