I updated my DriveSmart GPS map using Garmin Express. Express gives the option to also load the map onto my PC for use by Basecamp, and I selected that as well.
It turns out that Express also updates Basecamp and as a result, Basecamp no longer contains the 100s of trail lists I have accumulated over the years. That discovery was close to a heart attack that!
I then reinstalled Basecamp 4.6.2 on the PC, and all my 100s of lists are now present, life-threatening event averted!