Error: Ora-12560

ErroR: Ora-12560 TNS Protocol Adapter error...
Cause: This could happen when some one modifies the tnsnames.ora file ($TNS_ADMIN)
Action: 1) Check the connect descriptor
2) Check the host file name
3) See the service_name is correct specified is correct or not
4) See whether all the parameters are aligned in same way
Example of sample tnsnames.ora:
testdb =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = custprod)(PORT = 1521
(CONNECT_DATA = (SERVICE_NAME = testdb.world) ) )