ORA-27101: shared memory realm does not exist

Error:

Oracle docs note this about ORA-27101:

ORA-27101: shared memory realm does not exist

Cause:

Unable to locate shared memory realm

Action:

Verify that the realm is accessible

Solution:

In Windows may be to start the Windows services.

[or]

To resolve this issue with ORA-27101, you should be sure that ORACLE_HOME and ORACLE_SID are correct, and ORACLE_HOME should not have trailing slash.

For checking on trailing trash:
$ echo $ORACLE_HOME
$ /app/oracle/11204/ << Incorrect

$ echo $ORACLE_HOME
$ /app/oracle/11204 << Correct

For ORACLE_SID:
$ echo $ORACLE_SID
$ ora817

$ ps -ef | grep smon
ora_smon_ORA817
(Here, ORACLE_SID setting has been altered from ora11204 to ORA11204.)

Basically, to resolve ORA-27101, Once ORACLE_HOME and ORACLE_SID are correct, and then attempt to start SQL*Plus. 

Hope the fix helps you !!!!