Motherboard not lighting up when PSU is switched o

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I have an ASRock Fatal1ty Z370 Gaming K6 motherboard which is a quite recent board. While I was using my computer, everyone shut down rapidly. At first, I thought it was a power blackout in the house but the other appliances seemed to work. I then realised that the computer was refusing to boot up.

I thought that it was the power supply but after removing it, I was able to successfully power on an older computer that drew less power. The power supply is a 750W Corsair RM750X. I disconnected all hard drives, M.2 SSDs and memory DIMMs and decided to test the motherboard with the CPU and power supply separately. A video of the problem is available [URL=https://drive.google.com/open?id=1ASdFauVlQ85Elep4D0E96msVYf7qJOLS]here[/URL].

As soon as I flick the switch on the power supply, the motherboard lights up briefly before dying again. The on-board power button also refuses to work. If it may help, the BIOS was updated a week earlier but updated perfectly fine since the computer was being used normally and functioning normally before this incident occurred. Any help would be greatly appreciated.

I have tried that. Still the same problem. As soon as I connect the PSU, the lights turn on and then just die again. Power button doesn't turn on the system, with or without a CPU and memory. I can hear this buzzing sound from the sound card, where it says Creative Sound Blaster below the IO. I don't know what that means.