I hope you get help for this, I have fond memories of playing Demigod. It's too bad that it never really took off as a multiplayer game. I thought it had a lot of great potential.
Did you purchase the game from Stardock or from Steam? Can you check your "My Downloads" section of Stardock and see if they have your serial number there?
Sorry I can't be more help, I just loved Demigod, and I'm glad to see people still play it!