- Wait a moment and try to connect a second time.
- Check which services are currently known by the listener by executing: lsnrctl services <listener name>
- Check that the SERVICE_NAME parameter in the connect descriptor of the net service name used specifies a service known by the listener.
- If an easy connect naming connect identifier was used, check that the service name specified is a service known by the listener.
- Check for an event in the listener.log file.