I’m pretty new to diy PC repairs, and in April I came across a problem where my whole PC (5 year old pre-built) wouldn’t turn on at all.

Flipping the power switch and pressing the on button:
-Did not turn on any motherboard lights
-No whirrs, fans, or sounds of any kind from any component
-No leds

My first thought after some research was that it was the PSU, so I replaced it today (with an MSI A750GL PCIE5) only for the problem to persist.

I figure now that I know it’s not a power issue, that it’s probably the motherboard then, right?

Before I go forward with buying a new motherboard, I wanted to know what this loose cable was that I found while taking out the old PSU. It wasn’t plugged into anything, and it’s connected to this part that (I THINK) is also attached to the motherboard on the opposite side. It’s is also connected to the fans and the top on switch. However, I couldn’t find a place to plug it into. I don’t know what this part or cable is because I’m new to PC building and I’m working with a prebuilt.

In hopes that this magically solves my issue (since it seems to be connected to some important components), does anyone know if this cable is important/where it should be plugged into/if it should even be plugged in in the first place?

Update while typing this: I can’t seem to upload pictures of the cable (probably because I made this account just now lol), but it looks like a wide-monopoly house with 4 pins inside 1 hole, and the part itself is like tiny motherboard with a bunch of tiny wires coming out of it in a couple different directions placed on the rear-side panel of the PC.