Purchased songs have disappeared. From Music and iTunes for both Windows and my iPhone and iPad.

As noted:

If this content is then added back to the store by the new rights holders these will be different entries in the iTunes Store's catalog, and won't match to your old purchases.

That may not be the only explanation, but is is a possible one. The ability to redownload past purchases isn't guaranteed to remain in perpetuity. Contact Apple Support for more information. In the meantime see Backup your iTunes for Windows library with SyncToy - Apple Community and backup your library properly.

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We'll assume that you haven't changed regions, and have only ever used a single Apple ID. The ability to redownload or stream your past purchases depends on the continued availability of the items in the iTunes Store. Rights holders may remove content from the store without notice at any time. A typical reason for this might be a change of ownership of distribution rights. If this content is then added back to the store by the new rights holders these will be different entries in the iTunes Store's catalog, and won't match to your old purchases. Your existing copies and backups of such tracks would continue to play. If you haven't kept local copies and backups then you're in the same position you would be if you'd lost your 45 from Target.

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