During your first Microsoft 365 installation, you logged in with your personal Microsoft-account on the Microsoft website. You need to use the same account to re-install the software. You can do this by logging in with your account on this page: https://office.com/myaccount
If your Microsoft 365 product is linked to your Microsoft-account, you will see this in your Microsoft account under “Services & Subscriptions”. It will look like this:
Click on “Install Microsoft 365” to re-install Microsoft 365 Personal. After Microsoft 365 is installed, you will need to log in with the same Microsoft account within the software to re-activate your license.