I'm running Ubuntu 11.04 amd64 on a Dell Optiplex 990, which had the same problem and none of the solutions above worked for me. I found a solution in bug report in launchpad
Just add the option "reboot=pci" to GRUB_CMDLINE_LINUX in /etc/default/grub
to do this
look forCode:sudo gedit /etc/default/grub
and change it to
save, exit and update grub
Sorry for being bothering a solved thread.Code:sudo update-grub
I have Ubuntu studio 11.04 and it gets stuck at the very end of the shutdown process and I have to manually push the power button and it goes right off I don't have to hold it. What is strange is that it will restart just fine without any manual help. Other than that the OS works just fine.