This could be known and primitive to many people, however i am putting it to help the community in general from different technical-levels.

when you initiate sqlplus and trying to connect to your database instance you face the following error:

ORA-12560: TNS:protocol adapter error

if you are using windows OS, you need to check the following:

1. check that the database services is up and running, go to the search icon and search for “services” and ensure all Oracle services are up and running.
2.   if Oracle database is up and running fine, then check your environment variables in  CMD, you need to set the following:

set ORACLE_HOME=D:\app\db33\product\12.1.0\dbhome_33

set ORACLE_SID= ORACLE33

set PATH=D:\app\db33\product\12.1.0\dbhome_33\bin

to check that its reflected, execute the following to verify the value of ORACLE_HOME:

echo %ORACLE_HOME%

now try initiating sqlplus and it will be working 🙂