How to restore local music files after syncing with Apple music?

Did you install Apple Music the program? If so, uninstall it along with Apple TV or Apple Devices if those were installed as well. Reboot after and iTunes should have your music back. If you install Apple Music, it doesn't allow you to access the music in iTunes, but it will again after removing Apple Music.

Lets assume the following are true: You have iTunes installed, and have removed any of the apps Apple Music, Apple TV, and Apple devices, you are signed into your Apple ID in iTunes, your computer is authorized to your Apple ID, in iTunes under Edit > Preferences > General you have iCloud Music Library ticked, you have selected Music from the drop down list of media types, and View > All Music is ticked. You should now see your library, with any content that you have in the cloud and not stored locally, available to be right-clicked with a download option. You can select batches of tracks or albums to download. You also make a smart playlist that shows music that is not on this computer in order to highlight the content that needs to be downloaded.

See also Backup your iTunes for Windows library with SyncToy - Apple Community to preserve your library in future.

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