Question Recent PC build won't boot anymore. Help identifying problem?

Mar 15, 2020
Hi all, could use some help identifying what has gone wrong with a recent PC build. I suspect either the PSU or Mobo are dead, but can't identify which one.

Build Specs:
CPU: Ryzen 3950X
Mobo: Aorus x570 Master
Cooler: Corsair H115i Platinum
RAM: 32GB GSkill Trident Z DDR4-3600Mhz
GPU: Asus ROG Strix 2080Ti OC Edition
PSU: Corsair HX1000i
Storage: Crucial MX500, Samsung 970 Evo

This is my second build. I completed the build in mid-Jan, and everything has been working fine since then. Yesterday I spent some time playing Total War: Three Kingdoms and everything was running smoothly.

Today I went to turn the PC on and nothing happened. Usually, the mobo shows a red LED on the onboard power button at all times, and the GPU shows two white LEDs at all times where the two 8-pin power connectors are plugged. Both of these lights went out when I tried to turn the system on.
This hasn't happened before. When I tried pressing the power button again, nothing. These LEDs are now permanently off unless I unplug the 24-pin power connector to the MOBO, and then plug it back in.

I suspected the PSU might have died, but the HX1000i has a nice fan-test feature which I tried after removing all of the connector cables (so just the PSU plugged into the wall). PSU fan spins just fine. Also tried the paperclip trick with just the 24 pin cable plugged into the PSU, fan spins.

I've completely disassembled the rig and rebuilt one component at a time outside of the case, making sure all cables are connected securely. None of the components are damaged, there's no burning smell, no visible burns, no broken CPU pins or anything like that. Even with just the bare minimum (mobo, CPU, PSU, RAM) the thing won't start. The red LED on the mobo power button shows up when I reconnect the 24-pin cable, but goes out as soon as I press it to switch the thing on. This LED won't come back on until I unplug and reconnect the 24-pin.

Usually when I turn the system on, the BIOS fan and the GPU fans quickly spin up to 100% , and then calm down again once Windows loads up. Not sure if this information is relevant, but I wonder if the system has been pulling a whole load of power when it starts and maybe this time it killed something (PSU or mobo maybe).

Any ideas what my next steps should be, or can anyone figure out what the problem is here?

Help much appreciated (with the coronavirus thing ongoing, I need this machine so I can work from home starting tomorrow... )!
Cheers, Dan

Edit: One other thing, on the MOBO there's an LED backlit area (reads ESS Sabre Hifi) on the bottom left which lights up when switched on. When I reconnect the 24pin cable to the MOBO, this flickers on and then back off quickly. Quite suspicious the mobo might be dead...
Mar 15, 2020
One thing I hadn't tried yet was resetting the CMOS. I didn't really see how this might help as I haven't done any overclocking or made any big change in the BIOS.

So I reset the CMOS, and now when I reconnect the 24 pin cable the backlit 'Sabre Hifi' panel flickers lightly and there is a subtle buzzing noise coming from somewhere around the chipset (the PSU is switched on, but the pc is not). I'm pretty sure the mobo is dead.