I'm looking at a rez I made with the same kind of free night certificate and comparing the email from Hilton advising me that the certificate (Subject: Your Reward Certificate is Here!) is available to the rez on the web to the email confirming the rez had been made.

Reward Certificate ID from the original email is the same as:

"Reward Number" - While viewing the rez on the web, under the HH confirmation number.

"Honors Reward ID" - The email confirmation of the reservation

If you don't have any of these items available, then perhaps you can some of the Hilton folks to look it up in their records by supplying them with the field name.

ETA:

I just checked the Hilton website for a past rez made with the free night cert. Using the "Past Stay" tab, and expanding the detail, it contains the certificate number "CITI RC FREE WEEKEND NIGHT CERTIFICATE".

Although this may not be on your completed stay sine it sounds like the hotel didn't use the certificate.