Safe Ways to Force Reboot Ubuntu

(Credit: link)

If lucky,

CTRL + ALT + DEL

If still lucky, call a terminal

CTRL + ALT + F1/.../F5

If not so lucky,

ALT + SysRq (PrtScr) + K

Ah, everything fails. Here is the magic wand for Linux kernel

CTRL + ALT + SysRq (PrtScr)

That is to hold Ctrl+Alt, tap SysRq, then type in order:

REISUB

which is BUSIER in reverse order.

Reference

1. Freezes - Linux Guide - WikiBooks
2. Magic SysRq Key - Wikipedia