Once you have purchased a license for a unit of a Windows 10 operating system, you are well authorized to use it on the device till the support for the product ends (rather even after that).

However, the Windows 10 license is linked to the hardware of the device. In case you change any critical hardware (eg. the motherboard), the device would fail to recognize the license. Use the following procedure to rectify the issue:

Log in to the system as an administrator. Go to Start >> Settings >> Update & Security >> Activation.



It would read the message "Windows is activated with a digital license linked to your Microsoft account." or "Windows is activated using your organization's activation service." If it doesn't display this message, proceed with the following:

If it is a home computer (or a personal one), log in using your Microsoft account as explained below:

Go to Start >> Settings >> Update & Security >> Accounts >> Your info, and select Sign in with a Microsoft account instead.

Now return to the Activation page and check, your license would be activated.

Reference: support.microsoft.com/en-in/help/20530/windows-10-reactivating-after-hardware-change