Windows 8 Product Key

Hi! I have Dell Inspiron 3521 which was given to me by my sister. It has Windows 8 installed originally, then I upgraded it into 8.1. I used it for over a year since she gave me but then, it's original hard disk died. It was stuck for a year when I finally decided to buy a new hard disk but sadly, the hard disk didn't have an operating system. Now, I want to download Windows 8 from Microsoft and I'm aware that they'll ask for a product key. My question is, can I use the product key that came with this and where can I find it? Thanks!