Either broken psu or a short of some sort.
Remove all hardware not needed to boot into bios, even hdd, and remove all cables from the motherboard not needed to turn on, incl all fans exept cpu cooler.
Now use a paper clip (careful) to "shorten" the 2 pins the would connect to the power button on the case.
If it boots, its a short/ground problem, if it doesn't its most likely the psu.