Why would my computer shut off and not come back on

[momof2]

Hello my nephew was watching you tube today and while watching one of his video's, the computer totally shut down and we have tried turning it back on and even switching outlets to see if that was the problem and still no go! What would cause this to happen and how can we fix it if possible?

 

[Z1NONLY]

Bad power supply or bad motherboard would be at the top of my suspect list.