ORA-01034: ORACLE not available

There are many reasons for ORA-01034, some of them are listed below :

1. One of the most common reason for ORA-01034 is Oracle Instance is down.
2. ORACLE_SID or ORACLE_HOME or both are not properly defined.
3. Size of SGA is smaller than an Oracle Instance requires.
4. due to improper configuration of Listener , Remote user may face it.