Save ways to exit with system freeze

In Linux, when your entire system freezes, there are plenty of safe ways to exit and get back on track immediately. Alt + SysRq + (a selection of other keys) will do the magic trick. Note: 'SysRq' key is equivalent to the 'Print Screen' key.

Alt + SysR + K
Kill all processes (including X), which are running on the currently active virtual console.

Alt + SysRq + E
Send the TERM signal to all running processes except init, asking them to exit.

Alt + SysRq + I
Send the KILL signal to all running processes except init.

Alt + SysRq + L
Send the KILL signal to all processes, including init.

Alt + SysRq + S
Run an emergency sync (cache write) on all mounted filesystems. This can prevent data loss.

Alt + SysRq + U
Remount all mounted filesystems as read-only. This has the same effect as the sync combination above, but with one important benefit: if the operation is successful, fsck won't have to check all filesystems after a computer hardware reset.

Alt + SysRq + R
Turn off keyboard raw mode. This can be useful when your X session hangs. After issueing this command you may be able to use .

Alt + SysRq + B
Reboot immediately without syncing or unmounting your disks. Using this, you will likely end up with filesystem errors, so this is not highly recommended.

Alt + SysRq + O
Shut the system off right away.

If all these ‘Alt + SysRq’ keyboard combo are hard to remember, you can press Alt + SysRq + H to display a helpful list of the shortcuts above.

http://www.junauza.com/2009/01/linux-keyboard-shortcuts-to-exit-safely.html

Useful comments:
The first thing you should try in something hangs, is just Alt-SysRq-K (or Ctrl-Shift-Backspace while in a graphic X environment, which is a little less brutal). But mostly plain patience will do the trick.

A great way to use these keys is holding down Alt-SysRq while slowly typing REISUB (BUSIER backwards). This will reboot your system in a safe way (just look at the explanations above).

Also note: Alt-SysRq-H only helps you out in the command line.
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If you're having the screenshot (screen capture) try this; it worked for me in Ubuntu:
add this line to the end of /etc/sysctl.conf

kernel.sysrq = 1

then reboot.

For it to work properly, you need to press alt+sysrq simultaneously, then press your exit code (eg k) while holding the other keys down.
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Ctrl+Alt+Esc usually do the thrick for me ;)