The following tutorial will show you how to resolve the error in your Magento store e.g.:
Magento. Fatal error: Class 'Mage_Page_Block_Js_Cookie' not found in /home/p3r14781/public_html/store/app/code/core/Mage/Core/Model/Layout.php on line 465.
Note, the class may have different name. In order to resolve the issue, you need to download the version of Magento engine you have and open Magento folder in Total Commander (preferably). Then you need to go to Commands>Search (or click Alt+F7).
Copy the missing class name (you have seen it in the error message) and paste it to the "Find text" field in Total Commander.
Click on Start Search button. Few files should be found. Open the PHP file from the list and save it’s path.
Open the same path on your server and upload found PHP file from downloaded engine there
It will assk you if you wish to replace files. Click "Yes". Refresh your website.
Feel free to check the detailed video tutorial below: