Sometimes this happens. All of a sudden single sign-on no longer works on your web application. In my case I was able to fix this by moving NTLM authentication to the top of the list in IIS.
Here are the procedures:
1. Open IIS and select your default website
2. Click ‘Authentication’ and right-click on ‘Windows Authentication’
3. Select ‘Providers’ and move NTLM to the top