I recently had this happen with my silver-laced wyandotte roo, Gus. He was eating and drinking and acting pretty much like his normal self, the only clue that anything was wrong was when he tried to crow and nothing came out but a pathetic wheezy squawk.
I caught him for a closer look and realized that he was breathing through his mouth and wheezing. I separated him from the rest of the flock right away and started him on Tylan 50 injections (.75 cc twice a day cuz he is such a big boy). Within a few days he was back to crowing like his old self.