Official comment
Okay, now I see it ... I'm not sure if you did it accidentally or someone's having fun with you, but here's how to fix it:
1) You have the sub-browser visible. You can enable/disable a second view of the collection by clicking the bottom button on the toolbar at the far right side of the rekordbox window:

2) To get your left side tray back, put your cursor at the very LEFT edge of the track list, you'll see your cursor change to a double-headed arrow with a bar down the middle (the "resize window horizontally" cursor). Click and drag from left-to-right and the tray should appear once again.