I have Office 2010, but I lost the product key and now I can't activate it. Can you tell me where I can find or how to get a working key for this version?

There are several options you can try. First, if you bought Office through the Microsoft Store or another official seller, you can probably find the key in your account on their website. You can also try using a special product key office 2010. In any case, there is a way out and it is not so critical, and if I were you, I would use the second option.