If you use Joomla! 3.8.10 Stable and PHP Version 7.2 you might see an error “Call to a member function format() on null, Joomla admin page” on administrator default page or blank page only on administrator/index.php
Go to /administrator/index.php?option=com_plugins find Quick Icon – PHP Version Check plugin and disable it.
Now you can see your Control Panel.
Please update your website to the latest Joomla version.
But first, create a full backup of the site.
Check also the template and all third party extensions if they are supported in Joomla 3.9.