Hi
Is it a old Visio version or a version newer ? Newer version are linked to your Microsoft's account. Thus you can re-activate it by relogging inside the Visio activation's windows.
If it's an older retail version you had you need to run a key finder app on your old computer to retrieve the key used. You can't receover it with built-in application or registry. Key Finder from Magical Jelly Bean is one of such software, but you can find other's one.