Now What? Following a thread in PSUs . . . system won't power now

[BonzaiDuck]

If you want to look at the brief exchange, look for my recent thread in PSUs asking for a second opinion about cabling.

I fired up the system the first time, saw it wasn't booting but posting a message about "no system drive" or something connected. Entered the BIOS. Verified the drives weren't shown there (except for the ODD which was connected with a second SATA cable to the PSU.)

Found that I'd failed to plug in that six-pin cable to the PSU, corrected the problem, put things back together, connected power, VGA, NIC and USB keybd/mouse.

System won't power on from case-front power switch. Switched off. Checked AC cable connection.
Pulled side panel. Checked front-panel motherboard PWR, LED and RESET connections at the motherboard.

Switched on at PSU rocker-switch.

Green lights on the graphics card -- fine.
No error lights on the board.
Onboard power switch and reset switch (with red/green LEDs) are lit.
System will not respond to pressing the power switch.

Nothing had disturbed the very stiff and snug cable connections to the motherboard -- 8-pin and the main 24(?) Pin monster-cable-harness. Nothing.

Has my motherboard "gone south?"

 

[BonzaiDuck]

Most likely the PSU. Old one restored -- system fires up like a champ.

Thanks for wasting your time on my question.