These problems are usually very difficult to diagnose, the only advice I can give is try to boot with as few components as possible. That means unplugging cd/dvd drive, hdd, ssd, remove ALL cards from the mainboard (network cards, sound cards, video card etc). It would be great if you also have a onboard video card so you can use that one first. Also remove all but 1 RAM, and check in the mainboard manual which port you should insert it to.
Try to boot that way, if it doesn't start, replace or reinstall components like RAM, CPU, Mainboard, PSU etc. If it does start, just add one component at a time and boot up again after each install. That way you can easily see which component isn't working properly.