Menus disappear on update to 5.20.2 (Wordpress)
The definition of $civicrm_root was indeed the problem; in my settings.php (which had been working for a while) the path used Siteground username (as does DocumentRoot), whereas this should be replaced by the generic 'customer.'

How I got there: 

It was obvious that something was wrong with the Resource URL, but I couldn't figure it out (as the comments should make clear). So I reverted to 5.19.2, made a full-site backup, and then uninstalled CiviCRM completely: plugins/civicrm, uploads/civicrm, and the db. Then ran a fresh install to autogenerate a new settings.php file, which I downloaded.

Then I restored the full-site backup &amp; ran the CiviCRM update as normal, except that I put the newly-generated settings.php file in place before upgrading the db. This worked, fortunately, and the only difference between the settings files was the one described above.