You could check the paths in the config.core.php files in these directories:
§ MODX root directory
§ /manager/
§ /connectors/
§ /setup/includes/
Also, you could try going into PhpMyAdmin and repairing all the DB tables.
One ugly possibility is that someone has deleted some key files from the install. If there's a backup, you could try restoring the core, manager, and connectors directories.
As a last resort, you could back up the site and try upgrading to MODX 2.1.5.
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PLEASE, PLEASE specify the version of MODX you are using . . . PLEASE!
MODx info for everyone:
http://bobsguides.com/MODx.html