Check the "my products" section in your Adobe Account:https://accounts.adobe.com/products
If you've registered your copy of Acrobat with Adobe, you can retrieve the serial number from there.
For anything else, you will have to contact Adobe's support: http://helpx.adobe.com/contact.html (don't use any of the "Forum" options, otherwise you will end up here again)
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com