There could be a number of problems. One could be with the "fix" you initially used to take care of your web authentication issue, as jakkedup was questioning you about.
Another could be that you're logging in as a user that doesn't have permissions to view the hosts you're monitoring. By default, Nagios is configured only to allow username "admin" to view information pertaining to the hosts, so if you're logging in with the username "nagios," you'll be able to gain access to the nagios home page, but that's as far as nagios will let you go until you add that user to the comma-delimited list of users in your cgi.cfg file. If that is the problem and you act accordingly, you should be just fine (assuming that you've got everything else configured correctly).