Pardon interruption ... Howard is quite capable in assisting, but thought this might help ...
Suggestion ... sometimes one can get directly to an admin area by using a URL upon initial access to the server.
Without trying to login, go to:
https://yoursite/admin/settings.php?section=debugging
replacing 'yoursite' above with your server URL.
That should prompt you to login and then forward you to the page for turning on debugging.
Once that's done, click on Site Admin menu link.
Catch 22 ... if your moodle is in the middle of a failed upgrade ... the above suggestion might not work.
It's the only way I know of without ssh cli access to server or cPanel text editor where one could edit the config.php file to put server in debug mode.
Best of luck!
'spirit of sharing', Ken