Why did my garage door opener stop working?
the most common issue with a garage door that just suddenly stops working is the motor capacitor. Its an easy and cheap replacement part. a usual symptom if a capacitor going bad, is a garage door that starts to get slower to open. sometimes this gradual slowing happens at such a slow rate that most people don't start to notice until it finally stops working. 
 The best place to find a capacitor for you particular opener is inline or at your local Garage door repair shop. I can bet the repair shop will charge you FULL RETAIL price. I can't promise you that's whats going on with your particular situation, but I can say that most our issues with a sudden stop of an opener has been the capacitor. 
 I hope this helps. Good luck to you