A few months again my laptop stopped being able to shut down. I do not recall any changes which might have caused the problem. I was using Ubuntu 13.04 64-bit on a machine built by System76. I upgraded to Ubuntu 14.04 LTS 64-bit hoping this would solve the issue, but the problem persists. I can ONLY turn off the computer by pressing and holding the power button.
UPDATE OF PROBLEM: The unintended reboot only occurs when the laptop is plugged in, i.e. not on batter power.
The problem occurs when using the shutdown button on the upper right of the screen OR when I try command line options such as:
sudo shutdown now -P
sudo init 0
I have tried to solve it with the following:
sudo apt-get install laptop-mode-tools
but to no avail. I also tried booting from a live-cd into Ubuntu 13.04, then attempting the shutdown, again without success.