I received my code for ODST / free month of XBL, but I accidentally deleted the message / the message expired, what can I do?
If you make a post over in this Xbox Support forum, someone there will be able to help you out
For information: Xbox Live messages auto expire after 30 days. If you ever receive a code via one of these messages, make sure to either write it down or store it somewhere like a SMS message or a secure paste bin.
I got this from the Waypoint forums. All you need to do is contact Xbox support on the forum and they can help out.