ORA-00000: normal, successful completion

This error may occur in many circumstances you need to check environment variables:
$ORACLE_HOME
$ORACLE_BASE
$ORACLE_SID
also check /etc/hosts file it must contains correct hostname and ip.
example:

[oracle@oel6 ~]$ sqlplus / as sysdba

SQL*Plus: Release 11.2.0.3.0 Production on Mon Jan 27 11:55:17 2014

Copyright (c) 1982, 2011, Oracle.  All rights reserved.

Connected.
SQL> startup;  
ORA-01012: not logged on
SQL> shutdown immediate;
ORA-00000: normal, successful completion
SQL> shutdown abort;
ORACLE instance shut down.
SQL> startup;
ORA-00000: normal, successful completion

after that i checked environment variables:

[oracle@oel6 ~]$ echo $ORACLE_BASE
/u0/app/oracle
[oracle@oel6 ~]$ echo $ORACLE_HOME
/u0/app/oracle/product/11.2.0/dbhome_1
[oracle@oel6 ~]$ echo $ORACLE_SID
orcl

it's correct, then checked /etc/host file

cat /etc/hosts
#10.10.1.176 oel6

bingo! someone maybe system administrator or other dba commented this machine hostname and ip, hence we can't start database, uncomment it and everything goes correct.

vi /etc/hosts
10.10.1.176 oel6

sqlplus / as sysdba

SQL*Plus: Release 11.2.0.3.0 Production on Mon Jan 27 12:03:10 2014

Copyright (c) 1982, 2011, Oracle.  All rights reserved.

Connected to an idle instance.

SQL> startup;
ORACLE instance started.

Total System Global Area  835104768 bytes
Fixed Size                  2232960 bytes
Variable Size             645926272 bytes
Database Buffers          180355072 bytes
Redo Buffers                6590464 bytes
Database mounted.
Database opened.