It looks like a prolapse to me. Does she have an egg in there, with her insides wrapped around it? If there is an egg there, you will need to feel around to see where there is an opening to get it out. You also want to get her bum into some warm water (not hot, but warm). Put in some epsom salt and sugar (this is what my vet told me to do when it happened to my hen). Once her bottom is in the water and cleaned off a bit, feel around for an egg. If there is no egg, then you want to gently push the insides back in. Just make sure all the dirt is off.
After the prolapse is back inside, take your hen inside to a dark room. Keep her covered and quiet. You may want to keep her inside for several days in the dark to make sure she doesn't lay another egg before the prolapse has healed itself. My vet gave me some sugar water (kept in fridge) to rinse off the vent several times a day.
Don't let her dirt bath and get it all covered in dirt and debris.