There are two possible problems :
1. You do not have the default page set the web site
2. Directory listing is disabled
Solution :
1. Try to set the
default page
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.