Check that you've specified a valid service name with the SID
attribute in odbc.ini
.
If the service name is valid, it may be that the database service is not running or has not registered with the listener.
Failure to register with the listener may be a temporary condition. For example, the database may not have registered yet because the listener has only recently been started. It's therefore worth waiting a few moments and then reconnecting to your ODBC data source, before contacting your database administrator.