Where is it saying you have to buy the game again?
If you still have your CD Key, you can re-install the game with that and log in with your existing account.
If you don't have your key then email firstname.lastname@example.org
from the email address you registered in-game with and we can look up your key and username. You can use your key to recover your password in-game (it will send you an email) or we can reset it for you.
I've looked up your username and key and emailed them to the email address you registered with (same address as you use for the forum)