Did you purchase the game from humble-bundle, steam or the besiege website widget?

I presume you brought it through either Humble bundle or the website widget (which is connect to humble bundle). Now if you had a humble bundle account, then you would need to go back to the humble bundle website, linked earlier, and go to your library (click on your account on the top right) then from there you would be able to re-download it.

If you didn't have an account, you will need to find the original email from humble bundle, and go from there.