Just some minutes ago I decided to get some dust out of my pc. After I was done I put it all together and started my pc, nothing happend.I opened it up and had a look. When I pressed the start button on the front of my pc, all the fans started for like 2 sec, then stopped. (The little green light on the motherboard is working) I pulled all cabel out of my PSU besides my ATX and the other one (can't remeber what its called ) And tried again, this time, the PSU fan went crazy and started to go really fast so I turn it off so It wouldn't get destroyed or something.
Most likely a loss or forgotten connection double check everything including RAM and all data cables too. You can check to see if your PSU will power up by shorting out the green and black wires coming from the 24 Pin connector.