I'm so sorry to hear this. When I was little my parents had a cat that ran into a snowbank and broke a vertebrae and died. It was very tragic but there was absolutely, positively nothing we could have done. Cats will be playful cats and even if you had been home, you probably still would have had the same situation on your hands, although you would have been able to say goodbye. Please don't blame yourself, Nala is at peace and now that you know that it was an accident (accidents aren't really preventable) and nothing contagious hopefully you can be at peace too.