Explain your situation: let them know what ProduKey reports: maybe even the last 5 characters in Belarc if they are different than ProduKey.
If unsuccessful, call Microsoft 80 or 88. But if you honestly purchased Microsoft Office and misplaced the product key then no need to have panic attacks, there are some smart ways to get back the lost product key in no time. Hopefully it will get you to the point where you can re-install it on your computer. Product keys are generally required to verify the ownership of the program to avoid any kind of fraudulent usage. Unfortunately I don’t recall the steps you need to follow through but If you can get through the process chances are Microsoft’s assigned product key will be different than your purchased product key. The product key is stored in encrypted form in the registry and cannot be viewed in plain text after installation, whereas the Office version number is clearly displayed. Office 2010 and Office 2013 do not store their product keys in an easily accessible location in the registry file and many third party programs cannot scan for. Microsoft displays a product ID in the installed Office program, but this ID has nothing to do with the product key. Most notably, Microsoft Office 2013 and Office 365 do not store their product keys in the registry file. Even if you hadn’t used a Microsoft account and/or didn’t purchase it through the store, try anyway. However, it cannot find product keys for all programs. If you have a Microsoft account, try this.
In 2017 he had to re-install it and the following recommendation worked for him. I had used my Microsoft account to purchase it through the store. I installed Microsoft Office Home & Business 2013 in 2015 on a laptop I gave my son.