I'm in the process of upgrading to 12.04 (via command line, from 11.10).
Everything was downloaded and it was happily unpacking and replacing packages when my system froze.
I can move my mouse, but clicking on anything gets no response, and the display is not changing other than the mouse moving.
What can I do? I'm guessing simply holding down the power button until it turns off is a bad idea, but I don't know what else to do that would get things moving again.
(I couldn't find this question, but if it has already been asked and answered then a link would be helpful.)