For purchased content that has gone missing see Hide and unhide music, movies, TV shows, audiobooks, and books - Apple Support and Redownload apps, music, movies, TV shows, and books from the App Store, iTunes Store, and Apple Books - Apple Support in case it can be restored.
Conversely if purchased content that you have edited has reset to the original store metadata then edit your downloaded copies again and hide the original in the store so it won't undo your edits when it refreshes its view of your purchases.
See Backup your iTunes for Windows library with SyncToy - Apple Community for a way to backup the whole library and spot if/when any individual files are updated or deleted, so that you can choose to stop and restore those files if needed.
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