Lost Forum Passwords or Inactive Forum Accounts.
Lost forum passwords can be requested here
If you've changed your email address or have somehow had your forum account declared inactive, you requesting a new password will get you an activation code you can use to activate your account.
If you've lost the original email you used to sign up with for your account, you have to contact the admins with proof of identity if you want the account back. Alternatively you could just start a new account.