PC won't start - but all lights are on

I have my friends computer to reformat for him, so I went round to his yesterday to pick it up and it was on, so he shut it down etc.

I brought it back to my plugged all the writes in, monitor, power, keyboard and mouse, turned the power switch at the back on - the computer started for about 5 seconds and went back off, I didn't even press the power button on the front of the computer.

So I came to the front of the machine and pressed the power button and nothing happened, so I took the power cable out the back of the machine 5 minutes ago and when I plugged it back in the machine started up for a few seconds with the fans going and lights on the motherboard and then switched off again

Any idea how to fix this?
 
incorrect power supply voltage can cause that as can a short circuit....If it wasn't an issue before the first thing I would look for is a cable inside that may have come lose when you moved it. Would do no harm to disconnect all the cables from the motherboard and your Ram (clean with an eraser) reseat everything and see if it solves it.

I believe faulty RAM can also cause this to happen but it's too much of a coincidence to assume it went wrong between your friends place and yours.
 
This used to happen a lot to my old PC - but the fix was just to take out the RAM and put it back in again. I think it wasn't placed in properly.
 
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