@KIRAN1976
Ever since Office 2013, MS Office has been distributed in two different forms: (1) installed locally, (2) installed using Click-to-Run (CTR). The local versions used a product key received with the purchase (or via email, from a download), the CTR versions used a product key received via email from MS, but it was NOT stored on the PC.
If you have the Local version, you should be able to retrieve that key using these utilities: https://www.lifewire.com/free-product-key-finder-programs-2625119
If you have the CTR version, the key is not stored locally, and you can only get it by contacting Microsoft.
Good Luck
I am a volunteer and I do not work for, nor represent, HP