A post on PlayStation Network Support forums tells PSN users that if they can't log in, it may be due to a password reset.
"If you are getting an error that your PSN account password is incorrect, you may need to reset it. We routinely monitor for irregular activity, and if such activity is detected, we may sometimes reset passwords of affected SEN accounts to protect consumers and their account information."
In order to complete the reset, you will need your date of birth, and the email associated with the account.
You can read the full notice here.
And for details on how to reset your password click this link.