you haven't "put it back as was" - otherwise it would still be working.
If it worked OK before you started it looks as if you may have done something to the PSU, or to the motherboard. Even a tiny over-voltage on the motherboard will kill it stone dead.
I know you say that you've checked everything, but perhaps it's worth doing again - in great detail. Perhaps you could also let us have a little more information about what it was you did when you rebuilt the machine - presumably you added or changed some hardware?