Login to your account at magix.com, Account icon is top right of the home page, using the email address and password used the first time you registered the software.
Under My Products you may find your registered software, a download link and serial number.
However if this is a very old version, ie Sony branded, and was not a version transferred to Magix on their acquisition of some Sony products, then you will not be able to download and register/acitivate the software, the Sony servers no longer exist.
Hi @Don-Scott4578 and welcome to the Magix Sound Forge users community. If you have Sound Forge already on the PC but it will not open, try the 'reset & clear cache' procedure, On the Magix versions, there a 'File' menu command, but that menu item did not exist in the Sony versions. however, you can access the reset utility by holding the Ctrl+Shift keys whilst launching Sound Forge.
If you cannot download your older Sound Forge version from the 'Magix Service Center> My Products', most of the legacy versions can be downloaded from the Sony archives, but I am not exactly sure what Sony versions can still be activated through Magix. A friend of mine recently re-activated SF Pro 9 I think and I have SFP-10 and 11 activated on my Win 10 laptop.