ORA-01034: ORACLE not available ORA-27101: shared memory realm does not exist Linux-x86_64 Error: 2: No such file or directory

I get the following issue when connecting to Oracle DB via SID:

$sqlplus myusername/mypassword@sid
SQL*Plus: Release 11.2.0.3.0 Production on Wed Aug 22 00:35:56 2012
Copyright (c) 1982, 2011, Oracle. All rights reserved.

ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Linux-x86_64 Error: 2: No such file or directory
Process ID: 0
Session ID: 0 Serial number: 0

This server is running Oracle 11.2.0.3 on Redhat 6 64bit.

I made a change in /opt/oracle/112/network/admin/tnsnames.ora

# tnsnames.ora Network Configuration File: /opt/oracle/112/network/admin/tnsnames.ora
# Generated by Oracle configuration tools.

PSDS =
    (DESCRIPTION =
        (ADDRESS = (PROTOCOL = TCP)(HOST = oracle.abc.com)(PORT = 1521))
        (CONNECT_DATA =
            (SERVER = DEDICATED)
            #(SERVICE_NAME = sid)
            (SID=sid)
        )
    )

Then it worked!