Sounds like the jwhmcs-authentication was turned off or there's another authentication plugin that's overriding it. When it happened, did you change anything or do anything or was it just all of the sudden?
If you cannot login to Joomla admin, go into your hosting control panel to the database manager or phpMyAdmin -> jos_plugins -> Authentication - Joomla and change the "published" field value to: 1 then save! This will enable the Joomla authentication plugin so that you can login. Then, create a super admin user for Joomla and send us the credentials in a support ticket and we'll have a look and get the issue resolved.
Also, for good measure, DO NOT use the same email address for a customer in WHMCS as you would a Super Administrator in Joomla. These should not match.
The administrator has disabled public write access.