Hi and welcome to fpch
If you have changed the PSU for a known good one then I suspect that the problem lies with the motherboard.
Switch off at the wall - leave the plug in to maintain the earth / ground.
Take all cards ( RAM / Sound / Video etc etc ) off the motherboard.
Disconnect the power connections to the DVD and Hard Drives.
Disconnect the DVD and Hard Drive data cables from the motherboard.
Leave only the 20(4) way connector from the PSU connected.
Switch on.
You should, under normal circumstances, get POST beeps.
(assuming that your onboard speaker is working and you used to get one beep after switching on ).
If you don't get POST then your problem lies with the m/board ( assuming your PSU is good )
If you do get POST then put the cards / connections back one at a time until the problem is replicated.