I get the following error message when I try and checkout. The client wants this up and working by 7pm and I am freaking out! AHHHH

the error msg:

Not Found

The requested URL /zencart/index.php was not found on this server.

Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.
Apache/2.2.8 (Unix) mod_ssl/2.2.8 OpenSSL/0.9.8g DAV/2 mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635 Server at www.themileagemaximizer.com Port 443