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
This does not attempt to unmount or sync filesystems, so it should only be used when absolutely necessary, but if your drive is already failing then that may not be a concern.
In addition to rebooting the system the sysrq trick can be used to dump memory information to the console, sync all filesystems, remount all filesystems in read-only mode, send SIGTERM or SIGKILL to all processes except init, or power off the machine entirely, among other things.