Here are two ways to first try and kill just the process on your current terminal (thus allowing you to get back into your machine and at least attempt a shutdown -h now command) and if that fails, to bring your machine down in a more graceful manner than a hard shutdown.
Read it at arsgeek
There you are, staring at a crashed Gnome session, CTRL-ALT-BKSPC does nothing. ALT-CTRL-F1 wont bring you to a terminal where you could cd to /etc/init.d and restart gdm. In short, your choices seem to be limited to holding down the power button and chancing file system corruption or nothing.