There are two possible problems :
1. You do not have the default page set the web site
2. Directory listing is disabled
1. Try to set the
for your web site
2. Enable the Directory Browsing
Follow these steps to enable the Directory Browsing
i. Right click on virtual directory
ii. Go to the properties
iii. Go to the Home Directory Tab
iv. Check the Directory Browsing check box
Refer Directory Browinsg for pictorial represenation
Hope this helps.
All the best.