My first thought is a faulty power supply. As they age, they can deliver less power. So if your system peaks at 500 watts and your PSU is rated at 550, over time your PSU will be stressed as it can no longer reach it's rated power output and could fail, or even damage components in the system. If you can, try replacing it with a known good PSU. It's possible the motherboard has already been damaged as it's not powering on, so I'd not try again with that PSU.