The ctrl-alt-backspace key combination was used to restart X-server and was recently disabled because of a upstream (X.org) design decision:
There are two ways of still using the Ctrl Alt Backspace behavior:
1. Press AltGR + SysRQ + K instead (AltGR is the RIGHT ALT button and SysRQ is labelled "Print Screen" most of the times, and remeber to press and hold the keys in the in the right sequence, eg. starting with ALtGR, and ending with the K(ill) key).
2. I don't know if this works on other distributions, but on Ubuntu, you can edit you xorg.conf (/etc/X11/xorg.conf) and add the DonZap "False" value. That will make Ctrl + Alt + Backspace work again.
So to do this, edit the xorg.conf file by adding this:
The Ctrl-Alt-Backspace key combination currently "zaps" (hard-restarts) the X server, and thus loses any unsaved data in applications, etc. This key combination is also largely undocumented, so users (probably ex-Windows users) may press this key combination without expecting data loss. This spec proposes to follow upstream's lead and disable this key combination by default in order to prevent this usability issue from occurring in normal installs.
There are two ways of still using the Ctrl Alt Backspace behavior:
1. Press AltGR + SysRQ + K instead (AltGR is the RIGHT ALT button and SysRQ is labelled "Print Screen" most of the times, and remeber to press and hold the keys in the in the right sequence, eg. starting with ALtGR, and ending with the K(ill) key).
2. I don't know if this works on other distributions, but on Ubuntu, you can edit you xorg.conf (/etc/X11/xorg.conf) and add the DonZap "False" value. That will make Ctrl + Alt + Backspace work again.
So to do this, edit the xorg.conf file by adding this:
Section "ServerFlags"
Option "DontZap" "false"
EndSection
