I would recommend that you verify the following:
1. Configurations in your data source are correct.
2. If you were able to connect earlier, ensure that the DB server is up and running. If you are using any virtual machine for the tutorial, it might have to be started again
3. If the Denodo Server and the data source are on different machines, check the firewall/proxy setting. Also, there should be no network issues. You can test this by pinging the server which hosts the data source from the machine which hosts the Denodo Server
Hope this helps!