I have installed Oracle 11g on my Windows7 OS.
This was working fine but suddenly I got the error when connecting oracle from Sqlplus.

```
C:\>sqlplus

SQL*Plus: Release 11.2.0.1.0 Production on Sun Aug 24 00:14:32 2014

Copyright (c) 1982, 2010, Oracle.  All rights reserved.

Enter user-name: system
Enter password:

ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Process ID: 0
Session ID: 0 Serial number: 0

```

I have set the ORACLE\_HOME=C:\app\product\11.2.0\dbhome\_1 and ORACLE\_SID as orcl.

listener.ora 

```
#Network Configuration File: C:\app\product\11.2.0\dbhome_1\network\admin\listener.ora
#Generated by Oracle configuration tools.

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (SID_NAME = CLRExtProc)
      (ORACLE_HOME = C:\app\product\11.2.0\dbhome_1)
      (PROGRAM = extproc)
      (ENVS = "EXTPROC_DLLS=ONLY:C:\app\product\11.2.0\dbhome_1\bin\oraclr11.dll")
    )
  )

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521))
      (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))
    )
  )

ADR_BASE_LISTENER = C:\app

```

tnsnames.ora

```
# tnsnames.ora Network Configuration File: C:\app\product\11.2.0\dbhome_1\network\admin\tnsnames.ora
# Generated by Oracle configuration tools.

ORACLR_CONNECTION_DATA =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521))
    )
    (CONNECT_DATA =
      (SID = CLRExtProc)
      (PRESENTATION = RO)
    )
  )

ORCL =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = orcl)
    )
  )

```

UPDATE1

My init.ora file:

```
# Example INIT.ORA file
#
# This file is provided by Oracle Corporation to help you start by providing
# a starting point to customize your RDBMS installation for your site. 
# 
# NOTE: The values that are used in this file are only intended to be used
# as a starting point. You may want to adjust/tune those values to your
# specific hardware and needs. You may also consider using Database
# Configuration Assistant tool (DBCA) to create INIT file and to size your
# initial set of tablespaces based on the user input.
###############################################################################

# Change '<ORACLE_BASE>' to point to the oracle base (the one you specify at
# install time)

db_name='ORCL'
memory_target=1G
processes = 150
audit_file_dest='<ORACLE_BASE>/admin/orcl/adump'
audit_trail ='db'
db_block_size=8192
db_domain=''
db_recovery_file_dest='<ORACLE_BASE>/flash_recovery_area'
db_recovery_file_dest_size=2G
diagnostic_dest='<ORACLE_BASE>'
dispatchers='(PROTOCOL=TCP) (SERVICE=ORCLXDB)'
open_cursors=300 
remote_login_passwordfile='EXCLUSIVE'
undo_tablespace='UNDOTBS1'
# You may want to ensure that control files are created on separate physical
# devices
control_files = (ora_control1, ora_control2)
compatible ='11.2.0'

```

Still getting the error:

```
C:\>sqlplus /nolog

SQL*Plus: Release 11.2.0.1.0 Production on Thu Sep 4 23:17:00 2014

Copyright (c) 1982, 2010, Oracle.  All rights reserved.

SQL> conn / as sysdba
Connected to an idle instance.
SQL> startup
ORACLE instance started.

Total System Global Area 1670221824 bytes
Fixed Size                  2176328 bytes
Variable Size            1157630648 bytes
Database Buffers          503316480 bytes
Redo Buffers                7098368 bytes
Database mounted.
ORA-03113: end-of-file on communication channel
Process ID: 1916
Session ID: 5 Serial number: 5

SQL> conn shah
Enter password:
ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Process ID: 0
Session ID: 0 Serial number: 0

```

I appreciate someone can help on this.Thanks in advance.