No sound from speakers

So to begin with my head unit wouldn’t appear in this post options it’s a Sony mex 5300bt and my question is as follows. I recently upgraded my speakers and I had 3 working.. so I thought yeh okay I’ll get the forth one in and bobs your uncle, installed the forth then went to test it and all of a sudden I had no sound what so ever in my speakers, checked fuses, wires all good, head unit still powers but I just don’t get any sound :/ cause?