Resolve Oracle SQLPlus Login Error

Scenario:
When trying login to Oracle database encounter error: ORA-12560.

Solution:
Method 1: Ensure Environment Variables: "ORACLE_SID" been set correctly.

Method 2: Ensure Oracle Service and Listener are running.
(OracleServiceSID and OracleSID11203TNSListener)
If the services already started, restart the services and retry the login again.