Can you ping the server where the database is located? If not, that's a deal breaker.
After you get past that, you'll want to confirm that the SID or SERVICE you have defined in your connection is being serviced by a listener on the server. TNSPing can be used to confirm that from your machine if you have the Oracle Client installed.
If you have a local DBA or other experienced Oracle user, they might be able to help you.