Okay, Check to make sure all Cords are plugged in all the way. (Inside and Out of the PC)
If nothing is wrong there, try doing a Virus Scan with this tool:http://support.kaspersky.com/us/viruses/rescuedisk (It’s the best in my experience. And on a side note, I reccomend this because my friend had a virus that did something similar.)
Also, Try using a Boot CD of Linux (That Virus tool Should work.) and turn off you’re PC with that and see if it restarts. (Unplug the HDD while you are there. To take the software out of the equation.) If it does, It is most likely a hardware problem.
If that works in shutting it off with the Linux disk, (HDD removed) Try contacting Microsoft Support and see if they have a solution. (Cause It is a windows problem.)