Download and install the package found here : https://our.umbraco.org/projects/backoffice-extensions/nexu
https://www.nuget.org/packages/Our.Umbraco.Nexu/
Go to the nexu dashboard in the developers section and click the "Rebuild nexu relations" button. This will crawl all your content and rebuild the link relations. As this process happens in the background you can keep working without a problem and come back to recheck the status.