How to Safely Reboot a Frozen Linux System

Switch to a TTY

If your graphical session is frozen you can often switch to a TTY, assuming default settings this can be done with Ctrl+Alt+F1 to F6.

System Request (SysRq) sequence

Sometimes the system is so frozen, that the TTYs cannot be activated. Then you can use a System Request (SysRq) sequence. Often SysRq is invoked by the Alt + SysRq/PrtSc keys, sometimes the Fn key is involved too (in laptops),

While pressing these keys you press the following keys slowly.

R E I S U B - reboot

R E I S U O - poweroff

See this link for more details:

・ Magic_SysRq_key

This is a way to poweroff or reboot linux systems gracefully, that often works to keep the file system healthy (in contrast to hard poweroff).