PC powered itself off this morning and will no longer power on

Hi, I have an Asrock X870e Taichi motherboard with a Corsair RM1000x PSU, AMD Ryzen 9 9800x3D and RTX 5090 GPU.

All have been working perfectly for several months.

I left my PC on overnight, which I've done hundreds of times, and it was still powered on when I woke up this morning. But after I got out of the shower I noticed it had turned itself off.

I was on my way out to work so didn't look into it at the time.

Jump to midnight, I've just got home from an evening out and went to see if the PC would power on. I could see a red light on the back off the case (the clear CMOS button) and the onboard power button was lit white.

Pressing the case power button, nothing happened. I tried pressing reset then the power button again, still nothing.

I unplugged the PSU from the wall socket, held the power button to drain any charge then plugged back in. Still nothing happens when I press the power button. The LEDs on the CMOS and power buttons on the mpbo do light up still.

I started taking bits out of the PC one by one, starting with my GPU. No joy still.

I got it down to just the CPU in the slot with nothing else plugged in, not even RAM. Still nothing, no beeps or BIOS codes on the little display next to the power button.

I checked the CPU itself to make sure it hasn't burned like I've seen some do online. There are no burn marks, no bent pins, all my cables look fine, no strange smells etc.

Does anybody know what I could try next to try to find the culpret?

The parts should still hopefully be under warrenty, I bought the motherboard 24th November last year and the. PSU around the same time.