Joomla site access issue

Assuming you are using Apache, the DirectoryIndex directive controls what happens when a directory ending in "/" is requested. It sounds like index.php is not included in this. Typically, distributions will add index.php for this directive when the PHP Apache modules are installed (often within a configuration file that is included, not in the main config file).

You may want to investigate the DirectoryIndex directives (e.g., grep -R DirectoryIndex /etc/apache2/) contained within your configuration files to see what they say. It wouldn't hurt to add a DirectoryIndex index.php* to your virtual host declaration as this directive is additive, but this could be a symptom of a larger problem.