We are using model class named "Service". This class extends ActiveRecord::Base.
This error "undefined method `find' for Service:Module" was appeared after deployment on production server where we are running mongrel as web server.
Webrick on developer's machine works fine, so we started searching in google and here is answer.
Problem is in name conflict between gem mongrel_service (mongrel should work as windows service) and our model class Service.
So, try uninstall (gem uninstall mongrel_service) and everything should works fine.