According to this page on the KFC website, the company will no longer be honoring the coupons they issued for the free grilled chicken meal. You will be able to submit for a raincheck and they will send you a valid coupon.
If that is too much hassle for you and you have an El Pollo Loco location nearby, they will be accepting the unusable KFC coupons on Mother's Day.
Kudos to all of you who were able to take advantage of this deal before it went south.
Hat tip to Common Sense with Money.