What Rails version is required in config/environment.rb ? If it's lower than 2.1.0 then it's because Dreamhost removed it:
%> gem list rails
*** LOCAL GEMS ***
rails (2.2.2, 2.1.1, 2.1.0)
You can try to freeze the old Rails version into your app. The problem with that is that you need rake for this, and rake needs the Rails version that is required in environment.rb. For a workaround see here:
(It's a solution for Redmine but it should work for any app.)
Of course if you're adventurous and keen to keep up with the modern times, you could also try to upgrade your app to a more recent Rails version. There's a lot of good stuff in newer versions of Rails, but they're not 100% backwards compatible. Just change the Rails version in environment.rb to any of the versions installed on Dreamhost (see above, maybe start with the lowest) and look for errors and deprecation warnings in the Rails log.