PC not switching on

My pc does not want to switch on and yes it is plugged in correctly. I checked the powersupply and the screen, speakers, fan, etc. all works fine. Is it the motherboard or CPU and how can I test this?
The power button on the box gave some trouble and had to be pressed a few times before the pc would switch on. I also checked this button and all the wires are still connected correctly.
I would really appreciate any help I can get.