I had an instance where I switched out only a new cpu and new HS/fan. Should have been a no-brainer. Knew everything worked fine before starting.
Strangest thing, it would not power up, like your situation... would not complete POST... I tried many, many things.
Finally, I pulled out (that is, disconnected power and data connections) EVERYTHING except CPU, CPU HS/fan, 1 stick ram, and vid card (no HD/floppy/optical/sound card/NIC, etc. connected at this point).
Turned on the power switch, it powered up and POSTed. Shut down, connected HD, powered up... it POSTed. Connected the next piece of hardware, wash... rinse... repeat... until every single piece of hardware was added back. Hasn't had a problem since then.
I don't know why this worked (some theories, but really only speculation). If you're out of ideas, you may want to give this a go.