Computer Power Supply

This afternoon, one of my computers has been accidentally shut down as its power supply stop responding. I am wondering what may be the reason of its malfunction. It was used for 1 year already and I don't think its life is with that length only.

However, I know that there are other possible causes, such as:
overheating due to bad ventilation.
improper shutting down due to power outage or any other causes
over/lower voltage input
Anyway, we have nothing to do except to buy a new one and replace it. Its warranty has been expired already and I even don't know how to troubleshoot and repair it.