Computer unexpectedly freezes

[andreas85]

Even while the cause is unknown, this can help prevent further damage

Gentle shut down a suddenly hanged PC to minimize a chance of getting a broken filesystem and data loss:

[HowTo] reboot / turn off your frozen computer: REISUB/REISUO

Difficulty: ★★☆☆☆ This tutorial was created because people are using a hard power off during freezes, so this will teach you how to turn off your computer when everything is frozen using softer means. So whenever you see someone saying they had to “Power Off” or “Hard Reset” their system without doing a REISUB, please send them here! If you’re reading this in response to a question, please click the green link above this text to bring you to the full unabridged text of the tutorial…

Linux Magic System Request Key Hacks — The Linux Kernel documentation