If you previously had a username,  your username will have been transferred to this new forum system, along with your registered email address, postings, etc.

However, your password was most likely not transferred, and so you will need to request a new password using the 'Forgot your password?' link, also available here . This process will simply email a new password to the email address you gave us when you originally registered.
When you receive the new password at that email address, you can then log in (and then change the password back to your usual/old one, if you wish).

If you have changed email address since you signed up for your old account, you will need to sign up for a fresh account.