ORA-01012: not logged on at instance startup.

Caused : An unfathered shared memory segment still exists from previous startup instance.

Solution : Remove the shared memory segment from OS using sysresv and ipcrm -m <problem shared memory id>.

DCDB1$ sqlplus /nolog
SQL*Plus: Release 11.2.0.3.0 Production on Fri Jul 24 10:48:15 2015
Copyright (c) 1982, 2011, Oracle.  All rights reserved.
SQL> conn /as sysdba
Connected to an idle instance.
SQL> startup
ORA-01012: not logged on
SQL> exit
Disconnected
DCDB1$ sysresv
IPC Resources for ORACLE_SID "sblcbs" :
Shared Memory:
ID              KEY
28311559        0xffffffff
28311558        0xffffffff
29360133        0x4681b78c
Oracle Instance alive for sid "sblcbs"
DCDB1$ ipcrm -m 28311559
DCDB1$ ipcrm -m 28311558
DCDB1$ ipcrm -m 29360133
DCDB1$ sqlplus '/as sysdba'
SQL*Plus: Release 11.2.0.3.0 Production on Fri Jul 24 10:51:53 2015
Copyright (c) 1982, 2011, Oracle.  All rights reserved.
Connected to an idle instance.
SQL> startup
ORACLE instance started.
Total System Global Area 6.8413E+10 bytes
Fixed Size                  2238616 bytes
Variable Size            5.3687E+10 bytes
Database Buffers         1.4630E+10 bytes
Redo Buffers               93618176 bytes
Database mounted.
Database opened.