Check the network listener with "lsnrctl status" from the command line and make sure it is running (it will have its own service on Windows), and that the database has registered
a service name with it.
If the listener is up and running correctly, make sure there is no system firewall blocking access to port 1521.
Try using the IP address of the host instead of the hostname, or switch to "localhost" if everything is running on the same system.