First, try to start the PC without any USB devices connected. Maybe it's just a USB Device that causes trouble.
Otherwise I would start with the RAM, if you have 2x 4GB RAM take one of them out and see if the problem still persists. If so try the other one. Also try different RAM slots. If this doesn't help, swap out the PSU and try another one if possible. I had the same trouble as you have a few times in the past and in most cases it was either defective RAM or a defective PSU. Worth a try is also swaping out the Graphicscard.
In case neither did solve the problem you can reset the BIOS,sometimes it works wonders. It can have many causes,worst case would propably be the MB or the CPU. In one case it was just the power on Button on my PC Case that had a lose contact somewhere, of course I only found out after I tried everything else.. Changing the button was just a work of some minutes.