HMC key combintaions quated from http://radek-it.blogspot.com/

CTRL-ALT-F1: Switch to Linux command line; no login possible. If you then click on CTRL-ALT-DEL the system will reboot.
CTRL-ALT-F2: Takes you back to the Xserver window.
CTRL-ALT-BACKSPACE: Kills of the Xserver and will start a new -fresh- one, so you can login again.