I had that exact same problem! I have a massive library as well. Everything was gone! Granted the physical files were still in my iTunes folder it's just iTunes was empty! Music, playlists, apps, audiobook, play counts, iTunes settings, etc all gone! I have multiple iTunes backups that I make once a week or so. I tried using a fresh backup & I still had the same problem. I tried using the directions found in this Apple Support Document, and that didn't work for me. But I've heard it works for some people. I'd say try that. If you don't see your content after updating iTunes - Apple Support
If that doesn't work (it didn't for me), I found an old version of iTunes on the net (12.1.2) and, using terminal commands, was able to remove 12.2 & was able to downgrade. That worked for me. However, I had to use a backup of my iTunes library folder because 12.2 corrupted my primary iTunes folder. Fortunately my backup is only 5 days old and I'm only missing 1 app & 16 songs, which is no big deal because they're still in the currupted iTunes folder so I just drag & drop them into the new iTunes and that's it.
Hopefully one of these methods works for you. Good luck!