According to Dona Sarkar, who runs the Windows Insider Program, users who have been affected by the crisis need stop using their machine, and contact Microsoft directly. In a tweet, Sarkar said that technical support teams "have the tools to get you back to a good state."

#WindowsInsiders If you've run into the "missing files after update" issue for 1809/October 2018 Fall update, please call our support line. They have the tools to get you back to a good state. This build is no longer available to download manually: https://t.co/Ce9WVILknp pic.twitter.com/fvisQi1c8g — Dona Sarkar (@donasarkar) October 6, 2018

If you're in this position, then you can call +1-800-Microsoft or the local support number if you're overseas.