I have files I created nearly 30 years ago. How? I back up everything I consider important. My "years of work" never disappear.
ASC user turingtest2 help document: Empty/corrupt iTunes library after upgrade/crash - https://discussions.apple.com/docs/DOC-6561
Here are the two main methods. Method 1 does revert your library list (but not media files) to the state it was at the time of the last iTunes update but is probably better if you just updated the iTunes application and are having problems. Method 2 is best if you have not recently updated iTunes but are willing to accept loss of some metadata.
1) iTunes: How to re-create your iTunes library and playlists - https://support.apple.com/en-us/HT201561 - These instructions have you open iTunes with an old iTunes library file archived at the time you last updated iTunes. This will revert your library content list and appearance to what it was at the date of the last iTunes update. Newer media not in this list will still be in your media folder and you can re-add these as new items by dragging your media folders to the Automatically Add To iTunes folder.
A method 1 shortcut that seems to work with more recent iTunes versions - https://discussions.apple.com/thread/7105888?answerId=28484935022#28484935022 - However, I have not tried this and would want to make sure it didn't start creating additional files and folders inside the "previous libraries" folder that doing things the official way would avoid.
2) If you don't see your content after updating iTunes - https://support.apple.com/en-us/HT203225 - These instructions use a trimmed-down version of your library file to rebuild your primary library file. You lose some metadata (ratings, play count, date added) but the listing should be fairly up to date.
