My beautiful roo has died

I spent the day away from home yesterday. When we got back the chickens had put themselves up. The rooster was huddled on the floor with his butt hanging part way out the door. He did not move when I picked him up and put him on the roost with the others. I checked him again in the night and he barely stirred but he was alive. This morning he was sprawled out on the floor. I do not know when he died exactly but he did die. There is no sign of trauma or an attack. Nothing I can see. I am worried he was sick and now may hens are going to die too. Can anyone help? Give advice? How to treat the hens for an unknown illness? what to look for on the roo's body to get a clue? anything?