In some rare cases IIS or Windows updates can cause the Anonymous Authentication for the portal's website to change from the Application Pool or Portal's Identity account to the generic IUSR account.
This will usually result in prompts for credentials or the portal just not loading properly.
Luckily this is fairly simple to correct if you have access to the Web Server the portal is located on within Internet Information Services (IIS).
Firstly, open up Internet Information Services (IIS). You should see a screen like this:
Expend the section under Connections , then expand Sites and click the portal's website.
Then double click the Authentication under IIS.
On the Authentication page you'll want to right click Anonymous Authentication:
And click Edit here.
It's possible it may have been switched to IUSR
Switch this to "Application pool identity" and hit OK.
It may be possible to need to recycle the Application Pool if you try to load the portal and get the same issue.