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

Open command prompt and execute the below commands:

set oracle_sid=DATABASE NAME
sqlplus /nolog
conn sys/sys as sysdba
shutdown abort
startup

Sometimes, the database will not be mounted correctly, so we need to mount it manually. For that, shut it down and start it up then mount. Log in as oracle user, then run the following commands:

sqlplus / as sysdba;
shutdown immediate;
startup nomount;
alter database mount;
alter database open;

ORA-01034 and ORA-27101 normally indicate that the database instance you're attempting to connect to is shut down and that you're not connected as a user who has permission to start it up. Log on to the server 192.168.1.53 and start up the orcl instance, or ask your DBA to do this for you.

Make sure that your ORACLE_HOME and ORACLE_SID are correct To see the current values in windows, at the command prompt type

echo %ORACLE_HOME%

Then

echo %ORACLE_SID%

If the values are not your current oracle home and SID you need to correct them. This can be done in Windows environment variables.

Check out this page for more info

SQL> sqlplus "/ as sysdba"
SQL> startup

      Oracle instance started
      ------
      Database mounted.
      Database opened.

SQL> Quit

[oracle@hcis ~]$ lsnrctl start