It has happened to me at least three times (twice in Turkey and once in Trinidad).

My advice is not to write, as the problem lies with the hotel. I would start with the Diamond Desk (providing that you are speaking to a genuine Diamond CSR) and then proceed to Hilton Guest Assistance (888-240-6152). Use the "the Hotel has made a mistake" approach, and make sure that you get a case number.

Guest Assistance will then phone/write to the hotel, which will respond to you.

If you don't like the first response, go back to Guest Assistance and escalate your complaint.

Good luck, and keep us informed.