Here is what to do if users are normally logged in automatically to the Portal, based off their Windows authentication, but are now seeing the login screen.
1. Verify the SQL services are running.
Sometimes after applying Windows Updates to the server, the services do not re-enter the started status and must be started manually.
2. Make sure the first Default Document in IIS is listed as 'PortalShell.aspx'
3. Verify ownership of the Portal Framework database.
The Portal SQL user should be the owner of each Passageways database.