Constant short will not allow the system to boot, just touch them momentarily.
Double check all the modular connections, make sure one is not loose. If not..., bad piece of hardware could prevent it as well.
To eliminate the case as the issue, pull the board out of the case and set it up on a sheet of cardboard. Plug in only the basics to boot. CPU, 1 stick of ram, and 1 vid card (so you can see post) if it boots you have a standoff touching something it shouldn't. If it doesn't swap the vid cards and try again, if it doesn't do the same for each stick of ram.
Also while you have the board out look for burnt traces.
Pull the PSU from everything, get a paper clip and short the PSU between the green wire and any black wire. hook up a case fan. plug the PSU into the wall and see if it comes on. If it doesn't the PSU is shot. if it does could be the board. bum a PSU from a buddy and see if it will start the system. If it still doesn't. Its the board.
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