Upgrade not functioning

I have just successfully updated my WordPress to 3.8.1. On the ticker message in the dashboard it says “Thank you for Updating! Please visit the Upgrade Network page to update all your sites.”

But when I click on Upgrade Network, it shows a blank webpage https://iamzang.com/wp-admin/network/upgrade.php.

This is also the case with updating the plug-ins. In my Dashboard there is a notification of “31 Plug-in Updates” but when I click it shows the same blank https://iamzang.com/wp-admin/network/update-core.php

What’s wrong this this? How can I fix this?