check out or login brings blank page (500 error)

I had 1.3.9h and just noticed there were some issues when trying to add an item to cart and check out. So I was advised to upgrade to latest version. I went through install and everything was smooth, however if I go to check out or even login, the page just loads a blank page. No error, no site, just white page.

Anyone else having this issue? Any suggestions on where to start looking?

This is the string after my domain name on one particular checkout: index.php?main_page=checkout_shipping&zenid=4dbb3b1c2a4ef47d9fc1e689f156dae2

Thank you in advance.

A great place to start ..

http://www.zen-cart.com/content.php?124-i-am-getting-a-blank-page-%28or-blank-part-of-a-page%29

Here is one error we are getting:

Server error
The website encountered an error while retrieving https://www.mydomain.com/index.php?main_page=checkout_shipping&zenid=2450bedf0e7defad7f22f79ce51fb170. It may be down for maintenance or configured incorrectly.
Here are some suggestions:
Reload this webpage later.
HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfill the request.

Check your /logs or /cache folder for any error files - they will have more specific information than a general 500 error.

[06-Dec-2012 08:44:04] PHP Fatal error: Class 'rfq' not found in
/home/mydomain/public_html/includes/classes/shipping.php on line 59

I have an rfq.php file in shipping folder. Is there a way to check if any addons or modules were installed or added to site? Its been a few years since I had my hands in this and not sure what if any others have added to this site.

This is where I'd start: http://www.zen-cart.com/wiki/index.php/Troubleshoot_-_Diagnosing_Obscure_Issues