It is easiest to just start over.
- The first thing is make sure you are signing in with a Microsoft Account rather than a local account.
- Remove your current Groove application by starting PowerShell with administrative privileges and deleting the app using command: get-appxpackage *Microsoft.ZuneMusic* | remove-appxpackage
- Go to the Windows Store and reinstall the Groove Music app.
When you first start Groove Music it should automatically start loading your local files.