Magento：订单查看页面上的白页

Posted this on http://magento.stackexchange.com but I haven't gotten an answer.

I am getting this error for a while and I am clueless to what is wrong with it. Whenever I click the view order page, I get a blank page. Now I installed some code on my index.php that displays the errors.

This is the error that I get:

Array ( [type] => 1 [message] => Class 'Mage_Sales_Block_Order_Info' not found [file] => /app/code/core/Mage/Core/Model/Layout.php [line] => 491 )

This is line 491 (and context)

/**
* Create block object instance based on block type
*
* @param string $block
* @param array $attributes
* @return Mage_Core_Block_Abstract
*/
protected function _getBlockInstance($block, array $attributes=array())
{
if (is_string($block)) {
    if (strpos($block, '/')!==false) {
        if (!$block = Mage::getConfig()->getBlockClassName($block)) {
            Mage::throwException(Mage::helper('core')->__('Invalid block type: %s', $block));
        }
    }
    if (class_exists($block, false) || mageFindClassFile($block)) {
        $block = new $block($attributes);
    }
}
if (!$block instanceof Mage_Core_Block_Abstract) {
    Mage::throwException(Mage::helper('core')->__('Invalid block type: %s', $block));
}
return $block;
}

Seems nothing is wrong there (and why should it, its magento default code)

Now the next part is somewhat strange. I expected the class to live in app/code/core/Mage/Sales/Block/Order/Info I looked it up. The folders Order/Info didnt exist in my folder. Nor in the fresh install zip.

What is going on and more importantly, how can I fix this?

I hope you are using latest version of magento never mind even if it is old looks like you are missing some order block files try copying all the files properly. some times you may miss some files while you download the files from magento website check your magento zip file size that you downloaded.