Hi
After leaving my computer for half an hour, I came back to it with no power. The only sign of life is the 'on' button on the tower flashing orange instead of coming on green. I've tried turning it all off at the main plug socket for a while, but when I press the power button, it just flashes orange again. I've tried unplugging things at the back, but to no avail, and I've tried holding the power button in for a while, still nothing. There is no bootup or fan noise at all... help!
Thank you

It sounds like your computer sheet the bed... If it isn't powering on at all, that would be my best guess.

If you hear no spin up of the CPU fans etc that likely means your motherboard is toast. At least that's what would make sense to me with what info you provided.