Automatically Turned-off PC

Hello,
Good Day!

I just wanna know the reason behind the problem of my desktop computer...
When I turned on my computer, it will automatically turned off...
It's just the same problem of one of the clients here.
But the worst thing is, it will turn off anytime without giving a hint when it will turn-off.
Usually, it will turn-off after the Loading page of my OS...
But there were times that it will turned off after the display of the BIOS Information...
I really don't know how to fix that problem...
I really need your help...
Please help me...

Is it not the power supply of my PC...?