Steps to reproduce the issue

Upgrading from 3.8.11 to 3.9, process hung for hours.
Terminated by browser refresh, page goes blank.
All subsequent attempts to log in to backend produces a blank page.
Front end (website) is okay.

Expected result

Successfully log in to administrator backend.
Upgrade to 3.9 (latest)

Actual result

All subsequent attempts to log in to backend produces a blank page.
Front end (website) is okay.

System information (as much as possible)

enabled "$error_reporting = 'development';" in "configuration.php", then backend login attempt produced the following error message...
Fatal error: Class 'JToolbarHelper' not found in /home/appcedar/public_html/administrator/components/com_cpanel/views/cpanel/view.html.php on line 36

Additional comments

I did not backup before started upgrade.
Refreshed in mid-update after much "hanging"..