Then your server is not pointing to the public folder of your rails site or more likely you have forgotten to change to change the folder that your domain points to so that it goes to your apps public folder instead of the olde directory.
Possibly the simplest way to do this is to replace the directory that had your old site in it with a sym link to the your_app_root/public folder
What you want and what you need are too often not the same thing!
When your head is hurting from trying to solve a problem, stop standing on it. When you are the right way up you will see the problem differently and you just might find the solution.
(Quote by me 15th July 2009)