Hilton phone agents can see your certificate in your account without asking you for the certificate number, so you might be okay. You can check your reservation status online, and the reservation should have a "Reward Certificate" number. If that number is the same as your Amex certificate number, then you're in perfect shape, and you do not have to bring the certificate number to the hotel. If the number is different, then the phone agent probably used points from your account instead of the certificate, and you should call in to have that reservation cancelled then rebooked with the certificate. If there is no "Reward Certificate" number, you're probably booked with a cash rate, which you should likewise call to cancel and rebook. Good luck!