Reboot

To ensure that the system will reboot no matter what, I always do this sequence:

# echo s > /proc/sysrq-trigger
# echo u > /proc/sysrq-trigger
# echo s > /proc/sysrq-trigger
# echo b > /proc/sysrq-trigger

This requests the kernel to do:

emergency sync of the block devices
mount readonly of all filesystems
again a sync
force an immediate boot; you can also use o for poweroff.

See here for explanation of this feature.