The service management system has changed.
Every system management toolset has own utilities. The utilities that you are used to using are the ones that come with upstart, which are trivial shims for
initctl start and
initctl stop. But this is Ubuntu version 15. You aren't using upstart any more.
You're using systemd, and the service control commands are subcommands of
systemctl rather than of
initctl. So services are started with
systemctl start, stopped with
systemctl stop, enabled with
systemctl enable, disabled with
systemctl disable, and queried with
Services and service configuration have changed.
You've presumably followed instructions like the Serial Console How-To to turn the supplied
/etc/init/tty1.conf into an
/etc/init/ttyS0.conf. This is an upstart configuration file and it will simply be ignored by systemd. None of what you have learned from there applies to systemd, not even the concept of run levels, which is "obsolete" in the systemd world.
The systemd configuration file for a
getty on a real terminal device is
/lib/systemd/system/serial-getty@.service. This is a template unit, parameterized on the name of the serial device file. So the actual service name to use will be
serial-getty@ttyS0.service. You just enable/disable/start/stop it like any other service.
If you want an actual serial console, rather than just an ordinary serial terminal, then you don't even do that. systemd has a mechanism that automatically instantiates
serial-getty@.service whenever the kernel is told to not have its console on a virtual terminal.