If you want to restart your system, try reboot command.
And to troubleshoot the reason of the problem, you need to check the log messages, that what happens at the end,when the system actually needs to reboots, but it shuts down. Logs will give you a clue.
Here is an interesting article on difference of reboot command and "init 6"
In Linux, the init 6 command gracefully reboots the system running all
the K* shutdown scripts first, before rebooting. The reboot command
does a very quick reboot. It doesn’t execute any kill scripts, but
just unmounts filesystems and restarts the system. The reboot command
is more forceful.
So, should I use reboot or init 6? – neither! My advice is to use
the shutdown command. shutdown will do a similar job to init 6, but
it has more options and a better default action. Along with kicking
off an init 6, the shutdown command will also notify all logged in
users (logged in at a tty), notify all processes the system is going
down and by default will pause for a set time before rebooting
(giving you the chance to cancel the reboot if you realize that you
made a mistake).