Yeah what Crow said, I've had both happen at once, a serious power surge or shutoff at critical time from the power supply croaking could wreck the HDD, or pop one of the components on the mobo which is what happened to my backup PC I had running all the time, unless it is under warranty you should open it up and check all hardware connections. Could be something simple, could be catastrophic failure you never know.
This, if it shut off because of a sudden power outage some of your components could be damaged. You always want to have your PC on a surge protector.
No that bitch better be on a battery backup...to protect your from the real green meanie...(brown outs)
Undervolting happens more frequently and is less noticeable than a power surge. It will kill your components over time since if you live in a terrible area you may get them all the time.
As for TS steps. I would first disconnect the power supply from the wall outlet wait about a minute andt hen plug it back in.
Does any noise occur when you attempt to push the power button on the PC.
Once you plug in the power supply (put ear up to the back of the power supply) do you hear any ringing or whining noises? (Signs of a cap going or gone bad)
Do you have any motherboard lights on when the power supply is connected to the wall?
Try the paper clip test:
Disconnect all power connection and then jump your PSU with a paper clip.
Jump the 24 pin connector anything black to the green wire WITH IT UNPLUGGED!!
Then make sure its not touching anything metal (The paper clip)
Double check no power connectors are connected...Flick it on.
If the PSU starts spinning up and doesn't poop out you know that basic functionality on the PSU is still all good.
This does not mean the PSU is good, but it gets you further than where you were without breaking down the entire rig.
PLEASE READ: When you jump the connection DISCONNECT THE PSU FROM THE WALL!!!!
Do not let it touch anything metal in the case. The paper clip that is.
Good luck.