Hi, you will need to contact Microsoft Support. See: I lost or damaged my product key
"If you damaged your product key, but have a valid proof of purchase, Microsoft support may be able to replace your product key or provide other options. Contact Microsoft support for assistance.
If you believe the product key has been lost, stolen, or misplaced and you simply would like to buy a newer version of Office, you can click the button below to compare prices and options and to make your purchase."