What you are actually seeing
When a hen lays, the lower end of her egg passage briefly turns outward to deliver the egg and then draws back inside. A prolapse is when that tissue comes out and stays out — a glistening red or deep pink bulge protruding from the vent, sometimes small and neat, sometimes trailing and swollen. It tends to happen in young hens laying early, in birds laying very large or double-yolked eggs, and in overweight hens whose muscles are not up to the strain, though any layer can do it. Because the tissue belongs inside the body, every minute it spends outside it is drying, chilling and exposed to dirt and damage — and to the rest of the flock.
The signs, on the hen and around her
The protruding tissue is the headline sign, but there is usually a supporting cast. The hen often strains repeatedly as if still trying to lay, walks stiffly or sits hunched with her tail down, and goes quiet and withdrawn compared with her usual self. Laying stops. You may find blood smeared on her vent feathers, on other eggs in the nest box, or on the perch, and in a busy flock the first clue is sometimes a commotion around one bird rather than the bird herself. Look closely at the tissue: bright, moist and intact is a better picture than tissue that is torn, darkening, crusted with droppings or already pecked. Whatever state it is in, she should not spend another hour in the run.
Your first moves, in order
Separation comes first, before any examination. Other hens peck instinctively at red tissue, and a prolapse can be pecked into a life-threatening injury in the time it takes to fetch a box. Lift her out and settle her in a clean crate or box with soft bedding, somewhere warm, quiet and dimly lit — the dimness matters, because darkness discourages further laying and straining while she is treated. Offer water within reach. If the tissue is dirty, you can dab it very gently with clean warm water to remove droppings or shavings, but that is the limit of home intervention: do not push the tissue back inside, do not apply creams or powders, and do not return her to the flock to see how she goes. She needs a vet, and promptly, both to replace and assess the tissue and to judge whether she can safely lay again.
Getting poultry help without a scramble
The practical difficulty owners hit is that not every small-animal practice is confident with hens, and a prolapse will not wait while you ring round. This is the moment when having located a poultry-friendly vet in advance pays off — practices that keep backyard hens themselves, farm-animal vets who cover small flocks, and avian vets in larger towns are the usual answers. Searching the KinsLe Pet Services Directory for practices and services near you, before an emergency forces the question, means the hen in the box is travelling to help instead of waiting on your phone calls.