REISUB is an acronym used as a mnemonic device to help users remember
the Magic SysRq Key sequence that is best to use when trying to restart a
frozen system without risking damage to the file system. You hold down
SysRq+Alt and press the R, E, I, S, U, B keys one at a time, in order. This
performs the following series of actions, listed in order here, with the letter
corresponding to the command capitalized:
unRaw—Takes control of the keyboard back from the X server
tErminate—Sends a SIGTERM command to all processes, which allows
time for the processes to terminate gracefully
kIll—Sends a SIGKILL to all processes, forcing any that are still running
to terminate immediately
Sync—Flushes data from memory to disk
Unmount—Unmounts and remounts all file systems as read-only
reBoot—Turns off and back on again, restarting the computerIf you have to use REISUB, allow several seconds for each step. Be patient.
Doing it this way can save you from the heartache of lost data.
Fixing an Ubuntu System that Will Not Boot
Although it’s uncommon, it happens: Sometimes a system will not boot.
There are many reasons a system won’t boot. The goal here is to help you
discover one that may help you recover your system. The ideas in this section
are for computers that have had a working Ubuntu installation; however, they
may also be useful if you attempted an install that did not work. The options
described here are not going to help with troubleshooting computers running
Windows or other operating systems.
Checking BIOS
If your computer is unable to boot at all—not even from a known-good
bootable USB drive or live DVD—there are two options. It is possible that
you accidentally reset the boot devices and/or order in your system BIOS. If
making sure those settings are correct does not help, you may have a
hardware problem.