Computer won't turn on?

[andantesash]

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, any ideas whats causing this and how to fix it?

 

[andantesash]

I also meant to say, 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

 

[johnb35]

Try a different power supply first. Make sure all connections are tight on the motherboard.