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You shouldn't have lost all your music - it was likely that you updated to iTunes 12.2.x and didn't update rekordbox as well. Apple made a change to the XML and how iTunes allows other programs to access it, so Pioneer had to adjust rekordbox to match. Don't forget that rekordbox keeps several backups of your database in the event something goes wrong or crashes, you can recover from them if you don't open / close the program more than 3 times (there are 3 copies, the oldest one is removed each time the application is launched).
If you are looking to use rekordbox without iTunes, you can do this by either directly importing the music, or if you are wanting to have playlists pre-established, simply export a playlist or XML file from that other application, and import them to rekordbox.