I'm not 100% sure they'll give it to you but your only option is calling Sony. You'll more than likely need to know the Security Question and the Date of Birth to the account to get that kind of info from them.
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If you don't feel like calling Sony you can contact them via email during their operating hours which will allow you to do a live chat with a representative. I recently did that to get my old account back. Just a heads up, make sure you have your security question answer, date of birth, and the serial number of the ps3 you registered on.