The Windows 10 Media Creation Toolkit is back at https://www.microsoft.com/en-us/software-download/windows10 and social media indicates the upgrade is once again available via Windows Update. It would seem that all is right again since a few November update snags. Happy downloading!
P.S. You can also download the Windows 10 ISO. If you’re already running Windows 7/8 the Toolkit above is likely the best download experience.