Sometimes a Linux system has a serious problem while running, preventing you from issuing a halt or reboot command to the system, meaning that you will have to force the system to reboot. However, using the power button to forcibly reboot isn’t the best thing to do to the system because it can cause corruption of the system. There is a method that will make sure that all the file systems are unmounted and before rebooting the computer.
To issue this command to the system, press and hold the Alt key and the SysRq (the Print Screen button) then enter the key sequence R E I S U B, while continuing to hold Alt and SysRq.
In all but the worst of cases, this should safely reboot your suffering Linux system.