Likely power supply failure. If the fans run and the system doesn't boot, usually means that 5V+ is not working.
Only real way to verify that is with a volt-meter / multi-meter.
Then you would start looking at motherboard problems. Though typical response to non-boot is to remove everything that isn't necessary. So no drives, single stick of memory, remove any potential GPU (probably not in this situation) This will rule out a faulty component that is shorting out 5V.
Without known working hardware to test with you can't really check CPUs or Motherboards though.