Q: I bought food from a store. After I ate it, a new kashrus alert declared it non-kosher. Am I entitled to reimbursement?
A: This depends on the reason for the kashrus alert. If the food was declared non-kosher because of a Biblical prohibition (e.g. improper slaughtering, meat cooked with milk), then the seller must refund the full amount of the money, even if the food was already eaten. If the prohibition was Rabbinic (e.g. chicken and milk, terumos and ma'asros nowadays) the buyer can return the food; if the food was eaten, the seller does not have to reimburse you (Choshen Mishpat 234:3-4).
Later authorities rule that even if the prohibition was rabbinic, if the buyer did not pay yet, he needs to pay only the cheaper price of non-kosher food (Pischei Teshuva 234:1). Furthermore, if the item is declared assur b'hanaah, forbidden to benefit from (e.g. chametz that was possessed by a Jew over Pesach), the sale is null and void even though the prohibition is rabbinic, and the seller must return the money even if the food was already eaten (C.M. 234:4).