You can install Impulse and use that to get your game back. Impulse is basicly the same thing as Steam, a digital distribution system. As long as you have the serial, you should have no problem getting your game back.
Most games now a days have some way or another to get them back as long as you have proof of purchase; whether it be the box, a receipt, the serial, ect. Many come with services such as Impulse or Steam which let you redownload your game with no questions asked. Serial numbers are especially useful, since they are what is used to register a product with such services. So if you did not register before and have the serial number, register the game with Impulse and you will be able to download it.
