Dunno why it happened ... might have to do with how you upgraded.
but ... look at the config.php file for the moodle site.
$CFG->wwwroot = '
Should be the url to your site - without www stuff.
I just accessed and it doesn't redirect!
Suggest clearing browser cache and cookies ... and if you have ssh into server execute from cli purge all caches ..
or see if you can go directly to via URL:
https://yoursite/admin/purgecaches.php
'SoS', Ken