(this post is just for a personal reference)
If it locks up completely, you can REISUB it, which is a safer alternative to just cold rebooting the computer.
While holding Alt and the SysReq (Print Screen) keys, type REISUB.
R: Switch to XLATE mode
E: Send Terminate signal to all processes except for init
I: Send Kill signal to all processes except for init
S: Sync all mounted file-systems
U: Remount file-systems as read-only
REISUB is BUSIER backwards, as in “The System is busier than it should be”, if you need to remember it. Or mnemonically – R eboot; E ven; I f; S ystem; U tterly; B roken.
In case you like mnemonics: Raising Elephants Is So Utterly Boring, or Reboot Event If System Utterly Broken. I’ve also seen it as RSEIUB (Raising Skinny Elephants is Utterly Boring)