Ps4 doesn't post, 3 beeps

I have dismantled and put back together my (previously working) PS4, but now when I press the power but it IMMEDIATELY beeps 3 times and doesn't even try turning on. No fan start up, no disk spin, nothing.

People with a similar problem have proven it was caused by heat buildup, but I don't think that's my problem because the motherboard is cold to the touch and there is no hesitation before the beeping starts, as people with this problem usually have their system try to boot up, it gets too hot and THEN it beeps and turns off.

I have seen other answers say that it means there is some problem with posting, yet no suggestions as to exactly what the problem might be.

My guy is telling me that something isn't plugged in properly or something similar, but as far as I can tell everything is in tight.

Does anyone have any suggestions as to what this might be?