At the top of globals.php and globals_nonauth.php, add
ini_set('display_errors','On');
error_reporting(E_ALL & ~E_NOTICE);
If that doesn't work, and you have SSH access, I'd recommend installing the Composer package filp/whoops. 3 lines later, and you've got all the information you need.
I'd also recommend caution when using this as the default setup will expose raw code (perhaps lock the handler registration to your own ID/staff rank?)
If you don't have SSH access, or composer isn't an option, then check the Error Log. If the server on which the site is hosted is using cPanel, the Error Log can be easily accessed directly from within.
If that's not an option, then a logfile should be available within your system. Without knowing more about your setup, I can't really advise further