What happens if one of the testicles rotates 180 degrees vertically?

It depends,,,: The worst thing that can happen is something called torsion of the testes, where the testicle is twisted and so its blood supply is blocked. It is often accompanied by pain, tenderness and redness. This can be an emergency, and would need a doctor to reverse it, possibly in the operating room. Sometimes it is not this severe and can just go back into place on its own. Hope this helps.