I have ROG STRIX Z370-E Gaming with Win10. All was well for almost two years, then it suddenly stopped booting .... no CPU/chassis fans, no post, no beeps, black screen, no nothing, even when disconnecting the power button and shorting across the power switch pins on the mobo. I do note that the mobo "changing color feature" and power on led are lit when the power supply is connected to the mobo and turned on.

I removed the power supply (Corsair 750W) and did the paper clip test on the power supply by itself , and the power supply fan comes on as it's supposed to.

I removed the mobo, placed it on cardboard, and disconnected everything except the CPU and cooler (big Noctua heatsink and 120mm fan - I have not removed the CPU/cooler, as I've never had a CPU fail except due to overheating, and I seriously doubt that's the case here). I reset the BIOS and tried removing/reseating/changing memory, no joy. Questions:

-Presumeably the mobo could be bad - is there any definitive way to confirm that?
-*Must* the graphics card and/or boot disk be connected to test booting?

I note that this mobo is discontinued/unavailable, and I'm leery about buying a used one. Given that the my boot disk an Optane SSD with a cache card slotted into the mobo, what current mobos/chipsets can be used to replace it if that's the answer.