This can be one of two things. The files were uploaded to the wrong directory or the main file to the site is not named index.html.
All files must be placed in the www directory. If you have verified that your files are in the correct directory the the next step would be to verify that your main file is index.html or one of the acceptable extensions. If you are using FrontPage or Dreamweaver the program will typically set the files as index.htm. Here is a glimpse of the hierarchy:
index.html (Pole Position for you racing fans)
index.htm
index.asp
index.php
If you see any of the files above yours you will need to first delete it before yours will be seen. PLEASE read the Getting Started guide. You will aslo get other errors related to your site when your files are not named correctly or not in the proper directory. Linux Servers are case sensitive so don't use caps on your main file or you will get a permissions error.