Theres a forgot your password link on the sign on page. Hit that and you will have to answer a question or two (birth date, security question, etc.). They will then send your password to the e-mail address you used for your PSN account... If the needing the birth date is a necessity then you might have to phone sony and they'll ask you questions that should be easy enough for you to answer and easy enough for them to believe its your account... @
PheoNix