I had the problem again today, and I came across this possible solution, which may have worked for someone else.
I turned off "fast startup". I don't know if this will help or not. I have not been experiencing this issue often, so I won't know for a while if this helps or not until more time has gone by. I had this happen on November 20 and then again today.
I'm trying this option now. I will return when the problem occurs again (already with no expectations).