That is a huge prolapse. It looks much more wide than most pictures, so I would suspect that there may be an egg in there. Try to lube her vent and insert a finger to feel for an egg. Always keep the red vent tissue moist with some sort of oil. You may use coconut oil, Triple Antibiotic Ointment, olive oil, or vaseline, among others. If the tissue dries out, the tissue can turn black and die. She should be kept separated from others, as they will peck and damage her vent. Let us know if there is an egg felt.
Keep her vent clean, and let us know if she cannot poop. Soaking in warm water can help ease her to help her lay. A calcium tablet given orally may be helpful. Some prolapses can be caused by egg binding or constipation, but may also occur by itself. Hens who are overweight or who lay overlarge eggs can be at risk. Here is some reading about prolapsed vents or cloacas: https://the-chicken-chick.com/prolapse-vent-causes-treatment-graphic/