How to remotly restart/shutdown Linux box

Usual commands are:

sudo reboot
sudo shutdown -h now

But if those two don't work, you can use this as root:
- force reboot:

echo 1 > /proc/sys/kernel/sysrq
echo b > /proc/sysrq-trigger

- force shutdown:

echo 1 > /proc/sys/kernel/sysrq
echo o > /proc/sysrq-trigger

WARNING: This does not sync any disks, ....
You could also try

echo s > /proc/sysrq-trigger 

to sync disks before reboot/shutdown.
For all values of /proc/sysrq-trigger go to kernel documentation