Copying and pasting can result in 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.

If it’s not, then look for an open HTML tag and close it. Check your blog again and see if it is back to normal. If it’s still “messed up” then open the next post and do the same thing post by post until it goes back to normal.