If you registered the software, you can retrieve its serial number from the Adobe site.
Just go to Adobe.com, click on the Manage Account button and sign in with your Adobe ID. Then click on Plans and Products/View All and, on the next page, click the View Your Products link. All of your registered products and serial numbers will be listed.
Note, however, if you've used up both of your installations of the product, you may need to contact Adobe Customer Service and have them reset your serial number.