[Q] PC Start-up issues

Apologies if this is in the wrong forum, I haven't used this forum in years and there used to be a hardware specific sub-forum. Also the title is vague but I will go in to detail.

Had some issues with my PC the last few months, usually related to booting/starting up. Often it would "fail" to start up, it'd get to the "Launching Windows screen", my peripherals would light up then it'd instantly restart.

Recently however it hasn't been restarting, it's been completely cutting off. It'll get to that screen then just go blank as if the power was removed. Last night I noticed that if I launch with no USB connections at all, it'd launch fine, then instantly power off when a USB device so much as touched the port. Normally if I restarted a few times (usually around 10) it'd eventually boot and I'd have no issues unless a USB device were to come loose.

After last night however, it won't even boot up at all. I noticed the Motherboard light was flashing, so I removed EVERYTHING (all power cables, SATA cables, fans etc) and the light went back to being solid. I re-inserted each connection one by one and the light remained solid, even putting USB devices back in. But it still won't boot up. None of the devices appear to be faulty since the motherboard LED usually gives this away. The fans spin for a split second then go off. I WAS tampering with the BIOS a little last night to try and solve the boot issues before I knew about the USB problem, but I didn't change anything I didn't know what I was doing.

I'm not sure what else I could try, so any advice is appreciated. I got some good advice when I first built this PC a few years ago (yes, years) from here so maybe someone can help. I can supply system specs if required.

Thanks