My downloaded itunes song is corrupted and I need to re-download it

Delete the tracks that won't play from your library without hiding from your purchase history. Redownload using the cloud links that should remain, or close iTunes and reopen if they don't show up straight away. While downloading click the activity widget and turn off the option that allows for simultaneous downloads.

If that hasn't helped see If iTunes for Windows prompts you to authorize your computer when you try to play purchases - Apple Support.

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