If your system is totally locked up like that, there's one recovery thing to try:
Hold down ALT+PrtScr and then tap each of these keys individually a few seconds apart:
R S E I N U B
You can look up what all this does (each letter progressively starts shutting things down) but the short answer is that it flushes your buffers to disk, attempts to kill processes cleanly, etc., and the 'B' at the end tries a reboot.
There's a mnemonic to remember: Raising Skinny Elephants Is Never Utterly Boring
