I've never seen this kind of thing before! Have you tried switching to another console? If no reponse, you can try pressing the following 3 keys at once:
the left Alt key+ the PrintScrn/SysRq key+ R
This puts the keyboard in 'raw' mode. Then try Alt-Ctrl-Backspace or Alt-Ctrl-Fn again.
If that does not work, do the following:
the left Alt key+ the PrintScrn/SysRq key+ S (sync the disk)
the left Alt key+ the PrintScrn/SysRq key+ E (send termination signal to processes)
the left Alt key+ the PrintScrn/SysRq key+ I (send kill signal to all processes)
the left Alt key+ the PrintScrn/SysRq key+ U (umount filesystems)
the left Alt key+ the PrintScrn/SysRq key+ B (reboot)
This is called System Request sequence.