I going to surmise you are not getting any beep codes. Usually 1 long, 2 short which lets you know that DRIM pin issue usually. Since your machine was working and this isn't a burn in issue with new equipment it really comes down to some probably bad news things based on all the things you have checked off.

1) Bad ground on the PSU
2) Bad (24 Pin?) Cable to the CPU (Bent Pin Usually)
3) Bad pin on the mobo side
4) Bad CPU sector
5) Bad Mobo where its allowing power to CPU, but not POST b/c it has a short in the integrated circuit pathing


I thought you had said you swapped PSU's to make sure it was the PSU. If you didn't try another PSU than it could be a bad rail on your PSU or an internal short/ground in the PSU.

Don't make me have'ta Troll ya Bro!