A bit of Googling has provided some advice about restoring a backup of my template. At least I had saved a backup, that was a good thing; but the interface for the new whizzy Blogger templates then hid the admin interface so thoroughly that I had a struggle to find a restore option. Eventually that appeared, I pointed it to the saved backup, and this blog is now back more or less as it was a week ago. Whew.
Still, I'm not updating this site very frequently. You're more likely to get current news from Facebook and Twitter.