Good Evening,

I've come across a peculiar issue that pretty well removes my ability to work with my SharePoint server. The issue comes down to authentication.

I say the issue is peculiar because I'm able to log in to Central Administration interface, change site collection administrators, etc. But, when I try to log in to a site collection with the credentials I used to log in to Central Administration, I get the following behaviors:

• In Mozilla Firefox, endless authentication loop (type u/n and p/w, click ok, get prompt again.)
• In Internet Explorer, receive a message "the server declined to show the page." Details say that IE was able to get a response from the server, but the provided credentials are invalid.

To troubleshoot, I logged into Central Administration and set the site collection administrator for the collection I was trying to access to the same account as the SharePoint Farm Administrator. Alas, no luck.

I know that Central Administration and the Sharepoint : 80 systems use different databases and are different IIS App Pools, but I am truly stumped.

Are there any PowerShell commands I could try to restore access?

Are you trying to access your site collection from SharePoint server itself and are you using Fully Qualified Domain Name to access your site collection? 

If that is the case then you might be running into DisableLoopBackCheck issue.

If above is true then read all about this issue in this blog post from Spencer.

DisableLoopbackCheck & SharePoint: What every admin and developer should know

To fix this you can implement the Method # 1 as mentioned in the article below.

You receive error 401.1 when you browse a Web site that uses Integrated Authentication and is hosted on IIS 5.1 or a later version

Typically when you run into this issue you can access your site collection fine as long as you use http://SERVERNAME to access your SC.