APPROVED PROSEDURE WAS TAKEN BACK AFTER PROSURE WAS ALREADY DONE

I WENT FOR A PROSEDURE IN HOSPITAL. I GOT AUT FOR THE TREATMENTAND IT WAS APPROVED. 2 DAYS AFTER MY PROCEDURE WAS DONE ,DISCOVERY TOOK BACK THE APPROVAL AND DECLINED IT. AFTER I ALREADY WENT BECAUSE IT WAS APPROVED???? I AM NOW RESPONSABLE FOR HOSPITAL FEES, AND THE 2 DOCTOR'S BILLS. THEY PAID NOTHING. CAN THEY DO THIS AFTER A PROSEDURE WAS ALREADY DONE??????????