Have you got stuck up with this error in Oracle DB startup?? Here is the solution:
D:\oracle\code>sqlplus system/Admin123@xe
SQL*Plus: Release 11.2.0.2.0 Production on Tue Nov 7 14:44:36 2017
Copyright (c) 1982, 2014, Oracle. All rights reserved.
ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Process ID: 0
Session ID: 0 Serial number: 0
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
Enter user-name:
D:\oracle\code>sqlplus / as sysdba
SQL*Plus: Release 11.2.0.2.0 Production on Tue Nov 7 14:45:00 2017
Copyright (c) 1982, 2014, Oracle. All rights reserved.
Connected to an idle instance.
SQL>
SQL> startup
ORACLE instance started.
Total System Global Area 1068937216 bytes
Fixed Size 2260048 bytes
Variable Size 637535152 bytes
Database Buffers 423624704 bytes
Redo Buffers 5517312 bytes
Database mounted.
SQL> exit
Disconnected from Oracle Database 11g Express Edition Release 11.2.0.2.0 - 64bit Production
D:\oracle\code>sqlplus system/Admin123@xe
SQL*Plus: Release 11.2.0.2.0 Production on Tue Nov 7 14:47:02 2017
Copyright (c) 1982, 2014, Oracle. All rights reserved.
Connected to:
Oracle Database 11g Express Edition Release 11.2.0.2.0 - 64bit Production
SQL> exit
Disconnected from Oracle Database 11g Express Edition Release 11.2.0.2.0 - 64bit Production
D:\oracle\code>
