Hello,
Remember a one-time purchase version of Office is licensed for one install only.
If you're reinstalling Office, and you've already redeemed a product key and have an account associated with Office, then go to the section sign in to download Office.
If you don't have product key and have not activated Office under your Microsoft account, you can try finding receipt and contacting Microsoft customer service in case if they can help you with activation, but if you don't have even a proof of purchase, you can't do anything.