I cannot restart, reboot or shut-down my PC using Natty.
Whenever I hit shut-down, restart or in terminal
sudo reboot it shows a black screen.
If I should press down the power button for while, it is forced off.
Ctrl+Alt+Backspace will kill the X-server and bring you back to the log-on screen. You should be able to dismiss the session there using the shut-down / log-off / restart button.
If that fails to render the satisfaction of shutting down the machine then
Ctrl+Alt+F1 which puts you directly into the shell-propmt.
sudo shutdown -h now
or to reboot
This is more likely a bug of some sort that is causing this, which it has been happening more on 11.10 rather than Natty but much discussion has been focused on this occurrence and there are numerous threads on this site concerning the problem. Below is a link to one of the more in-detail discussion on the following matter.