Pc just won't turn on even after returning and getting a different PSU

I've plugged in and replugged every wire. I originally thought it was my Psu, I even removed that and plugged it by itself and it wouldn't turn on, thinking I had a broken Psu. The dude at the store tested and it said it worked fine but I still went ahead and bought a different brand. I've tried the new one and still nothing.. Can someone guide me on figuring out what the problem might me? How do I approach troubleshooting this?