ORA-01034

Quick fix

Cause

The session reached the server but there is no instance running for the ORACLE_SID (or service) it is using. Usually the database is simply down, or ORACLE_SID / ORACLE_HOME in the shell point at a different instance than the one that is running.

Fix

Check which instances run (ps -ef | grep pmon) and that ORACLE_SID matches one of them, including case. If the instance is down, start it with STARTUP as SYSDBA and read the alert log if startup fails.

ps -ef | grep [p]mon
echo $ORACLE_SID $ORACLE_HOME
sqlplus / as sysdba
STARTUP

Where it comes up