If your database was started in nomount mode you can encounter following error

SQL*Plus: Release 12.1.0.1.0 Production on Tue Jul 16 12:39:28 2013

Copyright (c) 1982, 2013, Oracle.  All rights reserved.

Enter user-name: sys@ora12c as sysdba
Enter password:
ERROR:
ORA-12528: TNS:listener: all appropriate instances 
are blocking new connections

To solve this problem you need to make static registration of your service in listener.ora.

Settings before solving the error

tnsnames.ora

ORA12C =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = laptop)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = ORA12C)
    )
  )

listener.ora

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (SID_NAME = CLRExtProc)
      (ORACLE_HOME = D:\app\oracle\product\12.1.0\dbhome_1)
      (PROGRAM = extproc)
      (ENVS = "EXTPROC_DLLS=ONLY:D:\app\oracle\product\12.1.0\dbhome_1\bin\oraclr12.dll")
    )
  )

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521))
      (ADDRESS = (PROTOCOL = TCP)(HOST = laptop)(PORT = 1521))
    )
  )

New settings

listener.ora

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (SID_NAME = CLRExtProc)
      (ORACLE_HOME = D:\app\oracle\product\12.1.0\dbhome_1)
      (PROGRAM = extproc)
      (ENVS = "EXTPROC_DLLS=ONLY:D:\app\oracle\product\12.1.0\dbhome_1\bin\oraclr12.dll")
    )
    (SID_DESC =
      (GLOBAL_DBNAME = ORA12C)
      (ORACLE_HOME = D:\app\oracle\product\12.1.0\dbhome_1)
      (SID_NAME = ORA12C)
    )
  )

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521))
    )
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = TCP)(HOST = laptop)(PORT = 1521))
    )
  )

ADR_BASE_LISTENER = D:\app\oracle\product\12.1.0\dbhome_1\log

You need to restart listener to make it effective. Once it’s done you should be able to connect to your database which is working in nomount.

SQL*Plus: Release 12.1.0.1.0 Production on Tue Jul 16 12:50:15 2013

Copyright (c) 1982, 2013, Oracle.  All rights reserved.

Enter user-name: sys@ora12c as sysdba
Enter password:

Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit
Production With the Partitioning, OLAP, Advanced Analytics 
and Real Application Testing options

SQL>

Have a fun

Tomasz