Sometimes apache can be configured if it doesn't found an index-page it refuses to show a dir tree.
You need to tell it which index-files to look for, so check in your conf if you have sumtin like this :

<IfModule mod_dir.c>
DirectoryIndex index.html index.html.var index.php index.php3 index.shtml index.cgi index.pl index.htm Default.htm default.htm
</IfModule>
Else try hashing out this one under here to test if it lifts the restrictions.

<Directory />
Options -All -Multiviews
AllowOverride None
<IfModule mod_access.c>
Order deny,allow
Deny from all
</IfModule>
</Directory>