Metadata expiry is the most common cause of SAML authentication suddenly breaking in my experience. Maybe the IdP or SP is signing messages with a certificate which had two years' validity.
Try updating the SP (i.e. Moodle) with the current IdP metadata.
Is the IdP still working with other SPs?
If there are still problems please tell us more about the environment, e.g. the Moodle version and which SAML authentication plugin is in use.