[lrossi]

After which you have to follow the straightforward systemd service troubleshooting procedure:

  systemctl is-system-running
  systemctl list-units
  systemctl list-units -t service --all
  systemctl list-units --state failed
  systemctl is-failed light.service
  systemctl status light.service
  systemctl show light.service
  journalctl -u light.service
  systemctl restart light.service
  journalctl -u light.service
  systemctl stop light.service
  SYSTEMD_LOG_LEVEL=debug /lib/systemd/systemd-lightd
  journalctl -u light.service
  reboot
  systemctl --force reboot
  echo r > /proc/sysrq-trigger
  echo e > /proc/sysrq-trigger
  echo i > /proc/sysrq-trigger
  echo s > /proc/sysrq-trigger
  echo u > /proc/sysrq-trigger
  echo b > /proc/sysrq-trigger

[majewsky]

Your shell must be very resilient if you manage to issue commands after SysRq+I.