When you try to log on to a Windows Server 2012 R2-based computer, the logon screen turns black after you enter the credentials. Then, the computer becomes unresponsive.
Note: The computer is unresponsive even if you press CTRL+ALT+DEL
Here is the fix: https://support.microsoft.com/en-us/kb/2897632
This issue occurs because an error occurs when the system tries to run the Userinit.exe process. Therefore, the Explorer.exe process cannot be started by the Userinit.exe process.