drishti,
Look for ORACLE_SID env variable setting on your server.
If there,s not ORACLE_SID value, ORACLE does not which databse server you are trying to connect on and so it's need to have it specified.
You can try to verify it running server manager.
Whent the prompt appears you should see the same tns error and if yopu try top connect as internal the same error arises again.
Then set instance parameter to you instance name writing
set instance <yourInstanceName>
and then try to connect internal without specifing the service name If it's running now you have to set the ORACLE_SID env variable.