You can see the MODX error log in cPanel's File Manager (or with FTP): core/cache/logs/error.log. While you're there (after you've copied the content of the error log somewhere), delete all files in the core/cache directory. It may be just cache corruption.
Check the modx_users table in PhpMyAdmin to see if there are any extra users, and see if the fields look OK for the admin user.
Check the modx_site_plugins table to see if there are any non-MODX plugins (core services is common in hacked sites).
Take a quick look at the root index.php and .htaccess to look for signs of hacking.
If there's no cPanel access, it's time to change hosts.