There are some cases in which you couldn't reboot a Linux system with reboot command or alternatives. If you have this problem and you don't have physical access to server, it is also possible to simulate pressing hardware reset button with the help of sysrq support in kernel.
To reboot the system through this magic interface, you need to activate sysrq first because it is disabled by default on all modern Linux distributions:
# echo 1 > /proc/sys/kernel/sysrq
After that you can send b trigger to reboot system immediately like that:
# echo b > /proc/sysrq-trigger
Please note that, rebooting with this method results to some data being lost. So, this method must be used only if there are no other chance to reboot system.