Lost Windows 10 Disc

https://support.microsoft.com/en-us/help/15088/windows-create-installation-media

Make sure you get the right one, this is not a quick way to upgrade as if wrong then activation will fail.

Step 1: How to download official Windows 10 ISO files

Step 2: How to: Perform a clean install of Windows 10

When you upgraded from a previous version of Windows or receive a new computer preinstalled with Windows 10, what happened is the hardware (your PC) will get a digital entitlement, where a unique signature of the computer will be stored on Microsoft Activation Servers. The Windows 7 or Windows 8 genuine license you were previously running will be exchanged for a diagnostics key.

Anytime you need to reinstall Windows 10 on that machine, just proceed to reinstall Windows 10. It will automatically reactivate. You will prompted to enter a product key a couple times through the installation, click I don't have a key and Do this later.

There is no need to know or get a new key, but if you have issues with Digital Licensing or the Activation Troubleshooter, you can utilize an existing Windows 7 or Windows 8/8.1 product key you already ownto reactivate if necessary. or use the reset function in Windows 10.