Microsoft has stated that once you install and activate Windows 10 on a system, you have a Windows 10 license for the life of that device, so yes, you do still have your Windows 10 License. @alphanumeric (a Microsoft MVP) can confirm this.
As Alpha says, you no longer need a key to install Windows 10. Just install using your Microsoft Account.
There's an app called "Showkey". You can find a tutorial here: Showkey - Windows 10 Forums but you still won't need it.