ORA-01034: ,ORA-27101: shared memory realm does not exist

Error Description:
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Whenever you try to connect to database it returns message,

ORA-01034 : ORACLE not available
ORA-27101 : shared memory realm does not exist

Cause of the Problem:
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This problem happens whenever ORACLE_SID or ORACLE_HOME is not properly set. Or for normal users whenever oracle database is not started. For remote users there may be a problem in listener.

Solution of the Problem:
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A)For Local Connections:
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1)Verify the ORACLE_SID and ORACLE_HOME is set properly. You can check the variable in UNIX by,

SQL> !echo $ORACLE_SID
data1

SQL> !echo $ORACLE_HOME
/oracle/app/oracle/product/10.2.0/db_1

If it is set incorrectly then set on UNIX by

$ export ORACLE_SID=db_name_here (on ksh,sh)
$ setenv ORACLE_SID=db_name_here (on csh)

Remember that ORACLE_SID is case sensitive on UNIX.

2)Verify the database instance is running.

On UNIX you can verify by,
SQL>!ps -ef |grep smon

It will return a row as ora_smon_your_database_name

If it does not return such row then your database is not started. So, start it after setting proper sid.

$sqlplus / as sysdba
$startup

On windows system press CTRL+ALT+DEL and see the process and look for oracle.exe. If it is not present there then from right click my computer>select manage>services>and select oracle database service and start it.
B)For Remote Connections:
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1)Verify the ORACLE_HOME is set correctly for all listed Oracle databases. If the ORACLE_HOME points to the different oracle version software, but the database was created using a different version, then this error can occur.

2)Review the listener.ora file and it is properly set. Like if you set ORACLE_HOME path inside listener ended by slash(/) then the error will come.

Incorrect:
(ORACLE_HOME = /oracle/app/oracle/product/10.2.0/)
Correct:
(ORACLE_HOME = /u01/app/oracle/product/10.2.0)

3)If listener.ora use SID then verify that you have set properly ORACLE_SID. Also be sure about listener services by issuing lsnrctl services.

4)Verify the database is running on server machine while you connect as normal user.

Other links
http://arjudba.blogspot.com/2008/07/database-startup-fails-with-error-ora.html
http://arjudba.blogspot.com/2008/05/startup-fails-with-oracle-error-ora.html
http://arjudba.blogspot.com/2008/05/database-startup-fails-with-errors-ora.html