Sometimes you may accidentally delete a paragraph or some imbedded html code in a page or post. You can easily go back to old versions (or drafts) of the page or post, and either reactivate that page, or copy the text you want from the previous version, and add it into the current version of the page.
When you’re editing a page, look for the Revisions section (should be a separate area away from the content area where you add/edit your content), and click on a version to compare it to the current version.