I would suggest repairing all tables in the DB with PhpMyAdmin and deleting all files in the core/cache directory.
Also, make sure you have this tag (with the exclamation point) in the head section of all templates:
<base href="[[!++site_url]]" />
As LK suggests, it's possible that your host has done something that changed the path to the files (making some of the paths and URLs in core/config/config.inc.php incorrect). In particular, check the connectors section.
Check the path in the three config.core.php files in the MODX root, manager, and connectors directories.