It looks like she may have FlyStrike. Did you notice any maggots?
Flush the wound well with saline, diluted betadine, chlorhexidine (hibiclens) or if you don't have that, warm soapy water (rinse well).
Apply Vetericyn or triple antibiotic ointment to the wound and place her where she won't be picked on and where flies can't get into the wound.
https://the-chicken-chick.com/flystrike-in-backyard-chickens-causes/
You may want to consider giving her a round of antibiotics. Do you have any on hand, if so, what are they?
Procaine Penicillin G can be found in the refrigerated section of most
Tractor Supply stores, that is usually the easiest to get a hold of. Dosage is 1/4cc injected into the breast muscle 1/4" deep for 4 days.
If she has not laid eggs in over a year, likely she has a reproductive disorder. She may have ruptured a little from fluid/mass in the abdomen or another hen pecked at her bottom creating a small wound that became infested. Does the wound seem to go deeply into her abdomen or is it more superficial? The worry is always that the wound is open into the body cavity.
Keep her hydrated and eating. She may benefit from poultry vitamins and extra protein as well.
Let us know how she's doing.
