My pc has no power unless the 8 pin is un-plugged

My pc wouldn't power on so when checking cables I found that the fans and LEDs would power on if the CPU 8 pin was unplugged which hasn't helped as I don't know what to do to get the pc to work.

I have a I7 7700k cpu if that helps and my pc has worked fine around 2 years previous to this and have not made any changes since.

Any advice would be greatly appreciated.

Thanks, Matthew