Run the Windows 10 Activation Troubleshooter: Start button > Settings > Update & Security > Activation and then select Troubleshoot.
Check if you have a paper copy of your Windows 25-character product key you can enter to activate Windows 10.
If your PC was recently repaired at an independent repair facility, contact them to see if the repairs require a new Windows 10 license.
If the PC is new and has never been activated, contact your PC manufacturer.