iTunes 12.2 "lost " library issue - https://discussions.apple.com/thread/7113917?answerId=28513415022#28513415022 - seems to work for some
First determine if the media files are still present in the iTunes > Media folder. If they are not you will have to restore them from the backup you of course made of your computer before installing new software. If they are still there then try one of the following methods to rebuild the database which tells iTunes what media you have in your collection and where it is.
ASC user turingtest2 help document: Empty/corrupt iTunes library after upgrade/crash - https://discussions.apple.com/docs/DOC-6561