To shut down a running Ubuntu system, you must not reboot with the reset switch on the front or back of your computer, or just turn off the computer. Ubuntu should be shut down in a controlled manner, otherwise files might get lost and/or disk damage might occur.
Alternatively you can press the key combination Ctrl+Alt+Del . If the key combinations do not work, a last option is to log in as root and type the necessary commands.
Just use reboot to reboot the system and halt to halt the system without powering it off . To power off the machine, use poweroff or shutdown -h now. The systemd init system provides additional commands that perform the same functions; for example systemctl reboot or systemctl poweroff.
 Under the SysV init system halt had the same effect as poweroff, but with systemd as init system their effects are different.