If your Linux system is frozen completely and there is absolutely no response and you are planning to unplug the power cable for hard reboot, there is a way to graciously reboot the system without unplugging the cable.
First of all ensure that your system is actually frozen:
Try Ctrl+Alt+Backspace combination if you are using graphical mode. If it doesn’t work, then try “Ctrl+Alt+F1“, “Ctrl+Alt+F2” etc to see if you can get to a shell in text mode.
If the Linux system doesn’t respond to any of the above attempts, you can do the following to graciously reboot the system.
Press the following sequence of keys with some time delay in between
R S E I U B
Each key has a special meaning after pressing Alt+SysReq.
R Take keyboard out of raw mode
S Synchronize the disk
E Terminates all processes
I Kill all processes
U Remount all file systems in read-only mode
B Reboot the system