Here is a little helper if your Webmin does not respond.

Just run this command from the command line and it will bring the webmin service up again fresh.

# /etc/init.d/webmin restart

Following messages will appear after running that command.

# Stopping Webmin server in /usr/share/webmin
# Starting Webmin server in /usr/share/webmin

After a fresh restart the  Webmin server should be running again.