Broody hen ate one of her eggs?
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It may be that it got accidentally cracked and then she ate it, rather than that it was deliberately broken open and eaten, so if it is an isolated incident I wouldn't worry too much. Check the shell to see how solid it is...maybe a thin shelled egg that was just a bit fragile for hatching.
Is she a first time broody?
I put food and water within reach of my broody on the nest (I use an old drawer for her nest and I hook food and water containers over the edge of it) The food is usually in the form of a wet mash with extra bits like scrambled egg, yoghurt or soaked mixed corn in it, as I is easy for broody hens to become dehydrated. I also let her out every day for a leg stretch, poop, dust bath etc. and if she doesn't get off, I lift her off and scatter meal worms or mixed corn to stimulate her to eat and move about and that stimulates her to defecate too of course..
Hopefully this is just a one off thing, but I would be inclined to lift her off the nest every day if she isn't doing so herself, at least until the last couple of days before hatch.