If you forgot your webmin password, you may need read following instruction which will show you how to reset it. Because I did it yesterday afternoon.
First, logon to the server via ssh and sudo as root user, or just ssh on server with your root user.
Second, say, you want to change or reset your password to [newpass].
/usr/libexec/webmin/changepass.pl /etc/webmin root newpass
After that, it will tell you，
Updated password of Webmin user root
Now you can login your webmin with your newpass.
It is tested under Centos 5.