There could be 3 reasons:

1. The home page doesn't exist.

2. The home page of the site doesn't have a valid name, the valid ones are:

index.htm
index.html
index.asp
index.aspx
index.php
default.htm
default.html
default.asp
default.aspx
default.php

3. The site has been uploaded in a subfolder, not in the primary folder, so the browser is not able to find it and open it. You have to move it in the primary space of the HTTPDOCS webspace.