I had this question after viewing Active Directory Broken
I had server working fine running server 2012. Its sole purpose was to handle incoming files via FTP. I installed SmarterMail 16 on it and configured IIS & the Windows firewall and all was fine. Everything worked or so I thought. A couple of hours later I tried to connect via a VPN to the server and received erroneous errors about the login authentication methods. Upon logging into the server directly I find Active Directory is no longer running. Directory Schema is unavailable. An invalid directory pathname was passed. Inappropriate authentication. I cannot get into any facet of Active Directory. I cannot get into Group Policy. I ran Dcdiag and LDAP returned error 52 unable to browse attributes. Any recommendations at this point would be greatly appreciated.