After adding AllowUsers xyz to /etc/ssh/sshd_config and restarting the server. I am unable to login with that user or either super user? To note that the access of the super user is still enabled in the ssh.Any way to debug this.
If you can get into the server, check /var/log/secure (Red Hat based) or /var/log/access.log (I think) on Debian based, that should tell you the problem.
If you can't get into the server, that's your first priority.