Thank you Leo,
You solved my problem. It must have been a combination of plugins that caused the problem.
I deactivated all plugins, then reactivated all the plugins that I wanted. It still worked.
Then I activated, tested, and deactivated each of the others in turn, and my left sidebar still worked.
That means that the problem was caused by the actions of more than one unwanted plugin. I can’t even remember why I installed most of the unwanted plugins.
Thanks again Leo.
Ian McAllister