ORA-01078: failure in processing system parameters

(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=db.rmohan.com)(PORT=1521)))
The listener supports no services
The command completed successfully
[oracle@db ~]$ sqlplus / as sysdba

SQL*Plus: Release 11.2.0.1.0 Production on Tue May 20 00:42:10 2014

Copyright (c) 1982, 2009, Oracle. All rights reserved.

Connected to an idle instance.

SQL> select name from v$datafile;
select name from v$datafile
*
ERROR at line 1:
ORA-01034: ORACLE not available
Process ID: 0
Session ID: 0 Serial number: 0
SQL> startup mount
ORA-01078: failure in processing system parameters
LRM-00109: could not open parameter file ‘/u01/app/oracle/product/11.2.0/db_1/dbs/initDB11G.ora’
SQL> startup
ORA-01078: failure in processing system parameters
LRM-00109: could not open parameter file ‘/u01/app/oracle/product/11.2.0/db_1/dbs/initDB11G.ora’
SQL> exit
Disconnected
[oracle@db ~]$ env|grep ORA|sort
ORACLE_BASE=/u01/app/oracle
ORACLE_HOME=/u01/app/oracle/product/11.2.0/db_1
ORACLE_HOSTNAME=db.rmohan.com
ORACLE_SID=DB11G
ORACLE_UNQNAME=DB11G
[oracle@db ~]$ cat /etc/oratab
cat: /etc/oratab: No such file or directory
[oracle@db ~]$ export ORACLE_SID=orcl
[oracle@db ~]$ sqlplus / as sysdba

SQL*Plus: Release 11.2.0.1.0 Production on Tue May 20 00:44:23 2014

Copyright (c) 1982, 2009, Oracle. All rights reserved.
Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 – 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> startup
ORA-01081: cannot start already-running ORACLE – shut it down first
SQL> shutdown
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount
ORACLE instance started.

Total System Global Area 3223535616 bytes
Fixed Size 2217672 bytes
Variable Size 1761610040 bytes
Database Buffers 1442840576 bytes
Redo Buffers 16867328 bytes
Database mounted.
SQL>