My post categories are empty. When editing or adding a post they are not displayed in the box. On the /wp-admin/edit-tags.php?taxonomy=category page none of the categories are displayed there either. If I attempt to add a category such as News which was previously there, I get the message, "A term with the name provided already exists with this parent." I have verified that indeed the categories still exist in the wp_terms table.
I have reinstalled WordPress 3.6.1 and unfortunately this did not fix the issue.