Normally Classic ASP shouldn't show detailed error messages to public users. Using the default settings an error on an ASP site will look something like:

An error occurred on the server when processing the URL. Please contact the system administrator. If you are the system administrator please click here to find out more about this error.

Clicking 'click here' won't help you - it'll show you a very general message.

To get more details you will need access to the server to change settings in IIS (Internet Information Services) or (depending upon the server setup) possibly FTP access which will allow you to download, edit and upload the web.config file.