After working fine in the morning yesterday, my laptop wouldn't start in the evening at our cabin. We cut short our stay so that I could check on what to do. I looked at several similar problems among user entries on the Apple forums, tried them, and had no luck. I called Apple Care, and even though my care expired about five weeks ago, because I had bought the latest OS last month, the tech walked me through some other steps. Nada!
Tomorrow I'm off to see if the Geek Squad can help at Best Buy. I probably should have had them come to the house to work in the computer's environment.
Meanwhile, I'll use my wife's computer when she doesn't shoo me away.