Any kind of animal protein (meat, fish, egg, cheese), but especially the ones I do not want to eat:
Earthworms. Maggots. Grasshoppers. The bit remaining on the child's plate after dinner. Whatever is left on the bones after the people finish eating. The egg that slid off the spatula onto the...
It's perfectly fine to feed starter feed to all chickens, with oyster shell in a separate container.
You could just put out a dish of each kind of food, although I suspect they'll all eat the starter feed if you do that.
Was that in the fall, when chickens naturally molt? (I don't know which part of the world you're in.) Did she act normal except for the not-laying?
Do you know for sure she's not laying? Because sometimes a chicken lays softshell eggs, that break and get eaten by the chickens. (If she's laying...