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https://www.backyardchickens.com/t/712322/poultry-fly-strike-a-cautionary-tale-with-graphic-photo-progression https://www.backyardchickens.com/t/832417/urgent-please-hen-has-huge-hole-below-vent-fly-strike-confirmed

Warm soapy water will get most of the maggots off. Give her some water with electrolytes and vitamins, and add some to her feed to make it moist. Give her a scrambled egg to eat. This link above that iwiw60 gave you has good instructions...

I agree that it sounds like maggots. Get her into some soapy warm water, and try picking off the maggots with tweezers. Permethrin spray, screw worm spray, or SWAT may be applied to the vent area to help repel flies. Fly strike can be deadly, so clean off vents in summer.