Being unable to authenticate using your valid SIP username and password is a general issue that might be resolved by some checks.
Verify that Mysql is running and listing using the netstat -apn | grep mysql command also check if the registration SIP requests are being processed by Mysql as quires by reading the Mysql log file using the tail -f /var/log/mysql.log command if all the checks are valid then you maybe have the domain part to be used for authentication in addition to the SIP username so verify that you domain name did NOT changed.
Also can you past what you see in the logs so we can help ? If Kamailio can NOT connect to the SIP DB or table the authentication will fail.