Checkout button is not displaying on the cart page and quantity is not updating on the cart page in Magento1.8 based website

I am using the Magento 1.8 version and am facing a sudden critical issue while checking out my LIVE website.

Problem: : The “Checkout” button on the cart page is not showing, and any order placement-related activity shows the below error:

Error:
Fatal error: Class ‘Mage_Payment_Model_Method_Cc’ not found in /public_html/app/code/core/Mage/Paygate/Model/Authorizenet.php on line 29

Debugged with the below steps:

1. I have checked the above file path the above class is being extended by
   “Mage_Paygate_Model_Authorizenet” in Authorizenet.php page.
2. Checked the admin settings in the Advanced menu.
3. Removed the Magento cache, Flushed the cache storage, and Checked the index management in the admin section.
4. Disabled the installed “One Step Checkout” module. But, still, the issue is coming up.

Please suggest me any further assistance, because it is very old version that’s why the support is not available.

Urgency: Client is asking for a solution ASAP.

Error Image: