PC wont power on after replacing PSU

Sep 30, 2018
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I bought a new PSU and tried my hand at installing it into my PC. It was a bit troublesome removing the old PSU, lots of ziptied cables and other components in the way. But I thought I had managed it. I only had to unplug the PSU cables, Hard drives, and remove the heat sink to get to everything. But after all that, it won't power on.

I press the power switch and get one red flash of the CPU LED on the motherboard and then nothing. I'm not really sure where to go from here and I can't afford to buy many new components. I'd rather fix it before buying replacements