ORA 12203 "TNS:unable to connect to destination"

Error Cause: 

 Invalid TNS address supplied or destination is not listening.
        This error can also occur because of underlying network transport
        problems.

Action Plan: 

Verify that the service name you entered on the command line
        as correct. Ensure that the listener is running at the remote node and
        that the ADDRESS parameters specified in TNSNAMES.ORA are correct.
        Finally, check that all Interchanges needed to make the connection are
        up and running.
1) Verify that you have entered the net service name you wish to reach correctly.
     2) Verify that the net service name ADDRESS parameters in the connect
        descriptor of your TNSNAMES.ORA file are correct.
     3) Verify that your TNSNAMES.ORA file is stored in the correct directory.
     4) Verify that the listener on the remote node has started and is running. If not,
        start the listener by using the Listener Control Utility.
     5) If you are connecting from a login box, verify that you are not placing an "@"
        symbol before your connect net service name.

Explanation:

        A client tool could not establish a connection over SQL*Net 
        to the named destination.

Note:

        Check for a client side log file (sqlnet.log) which reports the
        full error stack. The underlying error/s on the error stack may
        help identify the cause of the problem.

        If this is Windows NT and you get this when you go into Server Manager
        and also when you try and do connect internal make sure there is a
        service called OracleServiceSID and that it is started. If it does not
        exist use the ORADIM utility to create it.
    
        (Also check that your SID is set correctly in the registry.)