Get Back the License key for Symantec Backup Exec 8

I have a old windows server 2003 with Symantec Backup Exec 8. They are installed by supplier in 2009. Today we want to migrate the Symantec Backup Exec 8 License to a new server. but when I use the License key that provided by the supplier to install the software, it warning me the license key is wrong. The Supplier has lost contact and they have not get us the whole license agreement, How can i get back my true license key from my old server?