If /etc/nologin does exists, any attempts even popping out your emails, ssh connections, or webmail logins are completely denied. Just make sure you don’t logout from that server after issuing this command remotely or you have an active multiple ssh connections remotely to your server when you issue this command! If all fails, it would require direct physical access with your next ssh logins or tell someone to reboot the production server for you, which is bad.
Upon reboot, linux deletes this file automatically.
Any connection and authentication attempts are being
denied for now due to module authentication upgrade.
Please try back later after a few minutes. Thank you