It has happened to my two times already. Using the excellent front-end framework Semantic-UI for development is great, but when I have to deploy, is like I never used it!
You can find more astronauts in space than resources on how to fix this problem, but I managed to find this rather obscure article on precisely that.
It basically boils it down to:
Find config.assets.compile and true it up
#config/environments/production.rb config.assets.compile = true
compile them locally
RAILS_ENV=production bundle exec rake assets:precompile
all the git usual suspects
git add . git commit git push heroku master
I've just deployed 1 minute ago and it's working.
So, thanks Ellen!