Official comment
Hey Sebastian,
Sorry to hear that ... and yes, always backup, and I won't spare you these comments because it needs to be said for others as well. ;) Might I suggest now is a good time to consider something like Backblaze? ($4/mo for unlimited cloud backups that happen automatically.)
To recover the tracks, start with an empty rekordbox library, connect the export USB drive, find the playlists you want to import, right-click and select Import to Collection. This will also ask if you want to import the music - say yes, and give it a destination. If the songs on the drive aren't all in a playlist, you can create a new playlist on the drive, add all the songs from the drive to it, then import that playlist.
Let us know if you have other questions.