Yes, I have had 3 in about 360 hens have prolapsed so not a rare phenomenon. There is no cure, and the hen suffers horribly until she is put out of her misery.
I had one other small young hen which was laying very large eggs and appeared to suffer a partial prolapse. As soon as I found her I washed around her vent, and pushed everything back inside. I've never been able to catch her again but she seems fine, and has carried on laying quite large eggs.
Ross