Is it already installed on a system or on an old HDD ?? -- If so there are normally ways to get the product key off of the installation. (ie. Magical Jellybean Key finder )or in the windows registry -- If not then you probably can not get the product key if you do not have the label that came with it.
In either case Call the company's support and provide them with all the information ( where it was purchased and when, etc.) and they may be able to give you a new product key (depends on the CS rep you get and whether they believe or can verify the info you provide) -- worse case they will tell you that you need to purchase a new copy of the game.