How do I get my itunes library music back?

I pretty much fried my old Macbook Air by spilling water on it, I just got an iMac and am trying to get the music in my iTunes library back, what should I do? When I transfered from a Windows laptop to my old Macbook I didn't have to do anything and my iTunes music automatically showed up??? Please HELP
Update: I NEVER purchase music from iTunes, I always download it from the internet
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