Problem: Computer shuts down and starts up again.

Hi everyone, I built my computer a couple of weeks ago.

After a couple of weeks it's just started turning straight back on after I shut it down. The only way to get the computer to turn off is by switching it off at the wall.

Any idea on what could cause this all of a sudden? It's a Intel Core i3 2120, ASRock Z68M-ITX-HT system.

One other thing also, a few days before it started doing the previous issue, it has started constantly making the sound of something getting plugged into the USB and getting unplugged again. Any idea on what could cause this? The only thing I have plugged in via USB is a webcam, and it still makes the sounds with this unplugged. The mouse and keyboard are wireless.

Thank you so much for any assistance, it would be hugely appreciated.