Startup Fails With ORA-01012: Not Logged On

Problem Description
Users cant able to login to database server. Though it allow sysdba to login, it does not allow to run any query. Sometimes it will show database is connected to an idle instance. But when we startup database it shall throw error ORA-01081: cannot start already-running ORACLE - shut it down first.

 $ sqlplus '/as sysdba'

SQL*Plus: Release 11.2.0.3.0 Production on Sat Oct 12 07:53:11 2013

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

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0- 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> select instance_name,status from v$instance;
ERROR at line 1:
ORA-01012: not logged on
SQL> desc v$instance
ERROR:
ORA-01012: not logged on

SQL> startup
ORA-01012: not logged on


Changes
Oracle has been forcefully shutdown at OS level or crashed.

 CAUSE
An orphaned shared memory segment belonging to the ORACLE_SID still exists from a previous instance startup.

The command
ps -ef | grep $ORACLE_SID

shows no processes but with ORACLE_SID set the Oracle 'sysresv' utility shows a shared memory segment for a non-existing instance, e.g.

 $ sysresv

IPC Resources for ORACLE_SID "SUNIL

Shared Memory:
ID              KEY
5961234     0x00000000
5995678     0x00000000
6026789     0xb2e3c9ac

Semaphores:
ID              KEY
No semaphore resources used
Oracle Instance not alive for sid "SUNIL"


Solution
On OS level, remove the orphaned shared memory segment using:

ipcrm -m

$ ipcrm -m 5961234
$ ipcrm -m 5995678
$ ipcrm -m 6026789

$sqlplus '/as sysdba'

Connected to an idle instance.

SQL> startup Oracle instance started
Total System Global Area   1897667556566 bytes
Fixed Size                    18272817181 bytes
Variable Size               2732727182727 bytes
Database Buffers            467554367655 bytes
Redo Buffers                  19896776 bytes
Oracle Instance Started.
Oracle Database Opened.