If your machine has a disk error, it is possible that 'shutdown -r now' will never complete. That is when you can try 'reboot -f'. If that doesn't complete you have to kick it up a notch and utilize Magic SysRq.
First enable Magic SysRq:
echo 1 > /proc/sys/kernel/sysrq
Then use it remotely to reboot the machine at a very low level:
echo b > /proc/sysrq-trigger
No guarantee you will be able to remotely access that machine again, but it will be going down.
Thanks to http://blog.mohammadzadeh.info/index.php/howto-force-a-immediate-reboot