Upgrades from Windows 7 receive a Digital License.
If you ever entered a Microsoft account, your digital license may have been associated with that account automatically. If this is the case, then you might be able to use the Activation Troubleshooter to reactivate (assuming you have Windows 10 Anniversary or above). Otherwise, you may need to call Microsoft Customer Support.
Additional Information
Starting with Windows 10 Version 1511, it's apparently (potentially) possible to use a Windows 7, 8 or 8.1 key to activate Windows 10 in upgrades (though this excludes Volume, Multiple Activation or Enterprise Licenses).