PC wont Power-up


I'm using a 5 year old PC as a media pc which I have some problems with, the other day my PC would just not power-on, gave no sign of life.

After I had tried to turn it on X number of times there was life in the fans, etc. for about 2 seconds, then everything was turned off again - then there was no sign of life.

The first thing I thought was that the power supply was dead, so I bought a brand new power supply, plugged it in - but still it does not power-on. With the new power supply it's actually a new detail - On the mainboard a green light is on when power is connected, but nothing else shows signs of life. (Could also be that the green light was on with the older power supply but I can not confirm that)

There has been no lightning or similar in my area, not even rain a single day so the weather can't be the problem.

Is there anyone who has ever seen this problem?

What can the problem be?

what should I do to find out the problem?

Yes, it's an old PC but as I said, it's my Media PC and to buy a new PC is not my prior 1

Thanks for all your help! (I'm from Norway, so excuse my English)