I have migrated my Drupal 7 website to a server, and now I can't access the blocks via the admin menu; all I get is a white screen. It was working perfectly in my localhost.
If you get a white screen vs an access denied screen, the problem is a fatal PHP error, most likely a module or custom code fails when it's called about which blocks is provides.
You can get info about the error in your web server's error log.