Computer Switched Off by Itself - Motherboard/CPU dead?

[StealthRogue]

Two days ago, my PC just switched off with no warning. So I thought I switched off the power by accident. Pressed the power button and one of my cooling fan light turns on then off for a split second then nothing still happens. So I opened up my PC and pushed all connections in just in case one of them is loose causing a short circuit. Pressed the power button. This nothing at all happens. So the first component I wanted to rule out was the GPU -  since it was the easiest one to remove. Pressed the power button. Nothing happened. So, I ruled out the GPU. I repeated this process for my RAM, DVD Drive and HDD. I still get the same problem. So lastly, I switched the power supply (hoping the motherboard and CPU is fine). I get the same result. So now I'm left with either the CPU and motherboard. I hope it isn't the motherboard...

[truenorth]

It appears that you have chosen a good route to start at with the Power Supply Unit . However another easy test is to determine if all your RAM is working properly. Assuming you have more than one RAM module remove all but one and test for a change in performance. Also try the modules in different slots. Don't stop this process until you have checked all modules and hopefully one will be apparent as faulty. I take it to mean from your statement "I repeated this process for my RAM," that you only verified that it was "seated" tightly. It may be possible also that if a sensor is detecting a non functioning fan that it shuts down immediately as well. truenorth

[StealthRogue]

I don't know if this is important but, right now, the only thing I have in my computer is the power supply, CPU and the motherboard. Tested that. Nothing happens. I will try the RAM slots. None of the cooling fans are connected. The only one connected is the CPU one.

[patio]

I don't know if this is important but, right now, the only thing I have in my computer is the power supply, CPU and the motherboard. Tested that. Nothing happens. I will try the RAM slots. None of the cooling fans are connected. The only one connected is the CPU one.

Is it doing shutdowns in this configuration ? ?

[StealthRogue]

Is it doing shutdowns in this configuration ? ?

Which one? Having only the CPU, Motherboard and Power supply and no cooling fans connected (except the CPU one)? Then, it doesn't work.
For the RAM, advice from truenorth, it still doesn't work - trying each slot with a single RAM.