If your system is unresponsive there are a number of things you can try. This still works on Lubuntu 12.10
Restarting Ubuntu safely when it is frozen
In case of a freeze where you cannot do anything, you can use the following
Alt+SysRq R S E I U B
“Raising Skinny Elephants Is Utterly Boring”
keep in mind that the underlined keys must be kept pressed through the rest of the sequence AND that you will need to keep holding the sequence keys for a small period of time before going to the next one so that their actions can be carried out properly (For example, hold the R key for about 1-2 seconds before moving on to S). If the sequence does not work at first, then increase the time period between each sequence key press and try again.
Safely shutting down Ubuntu when it is frozen
Alt+SysRq R S E I U O
This is similar to restarting except for the last letter.
What is going on here ?
R – takes keyboard and mouse control from the X server.
S – writes all data from the disc cache to the hard-discs,
E – sends SIGTERM to all processes except init.
I – sends SIGKILL to all processes except init
U – remounts all mounted filesystems read-only
O – shuts down the system via ACPI or in older system
B – restarts the system without making steps to ensure that the conditions are good for a safe reboot, using this key alone is like doing a cold reboot.
You could also use ssh to remotely login and then use
sudo shutdown -h now