Power Failure during win 10 upgrade - now can't activate win 7

I have a Dell Optiflex 755 about 8 years old that was running Win 7 Ultimate. Microsoft downloaded Windows 10 and I started the upgrade.  At some point we had a power failure.  When power was restored the system restored to windows 7.

But now I get a prompt that "This copy of Windows is not genuine".  I have the product key but it will not activate. I get error message 0xC004E003 "The Software Licensing Service reported that license evaluation failed".  Microsoft says the key is valid but blocked by Dell. Windows 10 upgrade will not proceed unless windows 7 is activated.  How can II proceed?