We upgraded the Magento version from Connect Manager and the screen went blank. We couldn't login into our panel or even get out of the maintenance mode because we couldn't find the maintenance.flag file. Can you get us out of the mode so we can log in and get our site back up? Or is there another way?
Could you please enable error display in your index.php file and then send us what you get?
It might be because of php error.
You can try to check php log file or enable to show php errors at the index.php: