I am running DNN 5.0.0 with three different portals in it. I am using Windows authentication for logging in. This is a company intranet hosted locally on our server.
Two of my portals work just fine but for some reason one portal will no longer let me log in. When I go to the site, I get the Windows pop-up window to login and then I get the following error:

Windows Authentication Disabled.
Please contact your administrator.

This happened out of the blue one day and neither me (intranet administrator) or our network administrator did anything different. And neither one of us have been able to figure out how to fix it and why it isn't true for all the portals.

Any help would be greatly appreciated as we have been trying to figure this out for about a month now.

Thanks,
Dave

Thanks for responding!

I have checked the web.config and AD is enabled there and in the extensions page. I'm not sure what you mean by "installed in DNN"
From what I understand, the web.config file and the extensions page cover all portals in the installation so if there were a problem with AD being enabled in either of those two places, wouldn't it affect the other two portals in the installation?