Ahh yes... That error is very generic and vague. If you're unable to troubleshoot your application then I suggest putting in a support ticket.
Some things you might try is make sure you have your database.yml configured properly in your config directory. Same goes for your environment.rb file.
Recently we've made a change to how FastCGI handles Rails: http://www.dreamhoststatus.com/2008/03/14/fastcgi-environment-changes-for-rails/
Also you'll want to check your applications logs, most likely these logs will have the exact error.
- DreamHost Technical Support