If an Ubuntu machine become responseless

If the ubuntu machine locks up completely, you can REISUB it, which is a safer alternative to just cold rebooting the computer.
REISUB by:
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
B:  Reboot

REISUB is BUSIER backwards, as in "The System is busier than it should be", if you need to remember it. Or mnemonically - R eboot; Even; If; System; Utterly; Broken.