Hello Linux Geekstes.
In this article I will show you how to fix the shutdown issue on Ubuntu 12.04. If your computer reboots instead of shutdown, whatever shutdown method you try (both graphical and from the CLI), do this:
Open the grub file, /etc/default/grub, with your favourite text editor:
$ sudo vim /etc/default/grub
And change the GRUB_CMDLINE_LINUX_DEFAULT to look like this:
GRUB_CMDLINE_LINUX_DEFAULT="acpi=noirq quiet splash"
Your acpi may be turned off, so it will prevent your computer to shutdown.
It this did not fix the problem, install the laptop-mode-tools:
$ sudo apt-get install laptop-mode-tools.
laptop-mode-tools gets enabled automatically after installation and it may fix your shutdown problem.