Need help!

[Zencore112]

So I have a Z270 A pro (msi) and I also have a rx580 8gb i5 6600k and 16gb of ram, with a 256gb m.2 and a 2tb hdd, everything was working absolutely fine till I decided to be smart and change the psu from a 550w to 750w, after I correctly put everything in place I went to flick it on and it wouldn't start!! So I went through everything and figured out it was the gpu causing it! (No I didn't swap wires from old psu to new) so I decided to put the old 550w back in everything worked fine as it should and when I went to check my files my hdd wasn't being recognized, so I proceeded to put it in another pc wich it worked fine in but when I put in back in my pc it would work I changed the sata cables and made sure nothing was mixed from the other psu and still nothing... iv spent way to much on this beast and I'm about to go bananas! Only other option is updating bios if thst doesn't work would I have to replace mobo?