Computer won't turn on, blinks when power button pressed



My computer won't turn on. When I press the power button, the LED lighting at the back flashes, and the fan moves a tiny amount, but it doesn't turn on. It does the same when I release the button. I had put the computer to sleep the night before (as usual), and it just wouldn't wake up today. It was showing the flashing sleep signal on the power button, indicating it was asleep and not off, but it still wouldn't wake.

I've tried holding down the power button, and I've tried removing and replugging the mains power, to no effect. There is a green light on the motherboard, so I believe it to be a power-related issue, but I don't know. All USB-plugged devices have their lights on as normal.

This is a video of what happens when I press and release the power button:
I think it is worth opening the PC and checking the power cable runs and also the front panel connectors to the motherboard as these can move...Especially the power and reset cables from the case to the motherboard.

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