Won't power on after shutting down for first time in over a year

I have a Gigabyte GA-F2A88XM-D3H motherboard I purchased refurbished nearly 4 years ago to run my NAS with an AMD A8-7600 processor in it. It has been working great, but I was preparing to swap out the OS for a more up to date version and shut down the system completely for the first time in over a year before plugging in the USB to boot from (software initiated shutdown -h now from root shell).

From that moment on, nothing happens when pushing the power button with all the power connectors plugged in to the mobo and drives. Hooking up this power supply (and an alternate one I tried swapping in) to a PSU tester show both as working normally, all voltages a few tenths over nominal and the fans plugged into the PSU turn on. Still nothing after re-connecting the mobo.

I have another refurb board on the way but would like to get the current one back to working to have on-hand as a spare. Is there any further diagnosis to be done? What are my best alternatives for repair beyond the Gigabyte service center?