Think about the basics of how your web server works.
You have one file root folder (typically named www/ or public_html/ or something else depending on your setup) and you have sub folders and files within the root folder.
Web servers work so that they only allow public access to your "public root folder" and not your server files / language specific folders / system folders etc.
And now think about your error message for a second, it tells you that "You do not have permission to access this document." Which is basically talking about either your default.aspx or index.html or some other form of default/index file which loads when you load your root domain.
This is due to "permission error" on that particular file / dependent files etc, exactly as the error message states. You need to check the access mode of your files and folders, which in unix is done by chmod command, and in windows located in file/folder properties -> security -> Permissions which usually lists within a group of users, ie your public/all user group (if it exists)
You need these settings to give read or execute (exec depending on what type of file it is, if html/aspx than read is sufficient, strictly not write in any case as that allows modification to your files by the public) permissions in order for the web server to serve them to public successfully.
In order to give system specific help, your ASP.NET version is not enough to solve it, your IIS info is also needed as well as your server and file system setup etc