Lost local storage/Apple Music

iCloud Music Library is a feature of Apple Music and iTunes Match that matches or uploads your existing content so it can be available to your other computers or devices also connected to iCloud Music Library. Apple Music also lets you select additional content in the from the iTunes Store to add to your collection while your subscription remains active. Enabling iCloud Music Library should not have affected locally stored media, however that should nonetheless be backed up. Have you inspected the media folder to see if the content remains there yet is for some reason no longer listed in the library? iTunes doesn't automatically list the content of the folder, so if the database is corrupted and replaced with an empty database you'd need to either restore the database or reimport the media.

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Not expected behaviour. Without knowing exactly what steps you have taken it would be difficult to guess where things have gone wrong. Try Empty/corrupt iTunes library after upgrade/crash - Apple Community. Is there a complete backup of your library anywhere that can be restored, a device that holds all of your content, or something similar. Is your media stored in a non-standard location, e..g. on NAS?

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