cannot reboot linux server - magic SysRq key

The "magic SysRq key" provides a way to send commands directly to the kernel through the /proc filesystem.

 It is enabled via a kernel compile time option, CONFIG_MAGIC_SYSRQ, 
which seems to be standard on most distributions. 

First you must activate the magic SysRq option:

    echo 1 > /proc/sys/kernel/sysrq

When you are ready to reboot the machine simply run the following:

    echo b > /proc/sysrq-trigger