Did you see any evidence of an egg near her while she had the prolapse? Here is a thread from earlier today about a hen eggbound and with a prolapsed vent:
https://www.backyardchickens.com/threads/help-eggbound-hen-think-egg-is-sideways-or-too-big.1494825/
https://www.backyardchickens.com/threads/help-eggbound-hen-think-egg-is-sideways-or-too-big.1494825/