Hi,
How did you buy the Microsoft Office? If Office came with a box or card, look on the card or packaging to find your product key.
If you downloaded Office from an online store, the product key should be in your email receipt. Please check if you can find the email.
If Office came with your computer, look on your Certificate of Authenticity, or look for an Office product key card in your packaging.
If still no luck, I'd recommend you contact your local customer service for further assistance. For your convenience:
https://support.microsoft.com/en-us/gp/customer-service-phone-numbers/en-in?wa=wsignin1.0
Regards,
Steve Fan
TechNet Community Support