This is often a sign of database corruption. You have a couple options.
– You can try restoring from a database backup. If you do not have a routine setup to back up your databases, you should definitely do so. Some hosts provide nightly backups, if yours does not, then look at the plugins that do DB backups.
– You can try repairing the database tables manually. Start by switching to the default theme by renaming your current theme’s folder inside wp-content/themes and adding “-old” to the end of the folder name using FTP or whatever file management application your host provides. Then reset the plugins folder by FTP or phpMyAdmin.
Then repairing your database tables. Modifying wp_config.php to include the database repair doesnt seem to work often. You’d be better off logging into phpMyAdmin, selecting the affected tables, and running Repair Table.
You can find more information here.