You propably upgraded to Rails 4 from a 3.x or a previous version.
First generate a random secret key value:
$ bundle exec rake secret
Then take that value and put it in config/initializers/secret_token.rb:
YourApp::Application.config.secret_key_base="your-secret"
replacing YourApp with the name of your application.
The reason for this is explained here.
Also see http://guides.rubyonrails.org/upgrading_ruby_on_rails.html#config-secrets-yml