How do I stop Civicrm front end pages throwing a fatal error?
I tried many things, including restoring files from two months ago, and then updating, joomla and civicrm, and then reconnecting to the current database, and also rebuilding from vanilla copies of joomla and civicrm. But this did not solve the problem. The problem disappeared after I edited and saved the 'registered users' access rights in a minor way. The problem was still resolved after I set the access rights back to how they were. What a relief. Tony, many thanks for your reply.