Do not attempt to login using the password from the old forum / website. If you were a registered member on the previous incarnation of the site, then you can go to the login page, enter your username, and click the “forgot my password” link below the login dialog box. You need to do this even if you did not actually forget your password!
Assuming the email address you had used to register on the old site is still valid, you will receive an email with instructions for setting a new password (which can be the same as your old password if you like).
If you attempt to login unsuccessfully, using your old credentials, more than a few times, you will be temporarily locked out of the site, as would anyone trying to guess someone’s password over and over again.
Sorry for the inconvenience,