There is no obvious indication when shutting down Windows 10 that the "fast shutdown" feature was placed into effect. Previously, I had turned it off at least a couple of times. Since it does not happen often, I forgot how to identify and solve this issue. I spent a couple of fun filled hours dwelling on this before realizing what happened. A tutorial concerning how to fully shutdown Windows 10 is located here. How to fully shut down windows 10. An additional article for additional thoughts: The Pros and Cons of Windows 10’s “Fast Startup” Mode.Following the advice of that tutorial solved the problem and Ubuntu once again boots successfully. Should you have the same problem, I hope that his helps.