Click "Post New Topic" or "Post Reply", then click "If you have previously registered, but forgotten your password, click here" down under the page.
In the next page enter your email address you were using with your previous account and click "Request User Name/Password Now".

Soon after that you'll receive an email with your user name and password.

What I don't understand, however, is why you didn't change your password to whatever you like. This BBS works perfectly without cookies. I'm being here with cookies disabled, and still have no difficulty. (You have to give up "New topics here in this forun!" functionality, though.)