Active Directory Exception :The server is not operational

 hi all
on my login screen i am using ldap to authenticate user name and passowrd against active directory..it was all working fine till yesterday
but suddenly this mornin it has started throwing error
"The server is not operational"
the function which was working till yesterday is this
  Dim authentic As Boolean = False
        Try
            Dim entry As New DirectoryEntry("LDAP://" & "DomainName", txtLogin.Text, txtPword.Text)
            Dim nativeObject As Object = entry.NativeObject
            authentic = True


        Catch ex As DirectoryServicesCOMException

        End Try
 &nb ...