Sneezing can be a one off way for your dog to clear its nasal passage of foreign material. It is a normal bodily function and you do not need to worry if your dog sneezes.
Reverse sneezing is also a normal occurrence that most dogs will experience at some point. If your dog has a reverse sneezing episode, try to remain calm and allow the episode to pass. Your dog will not have any adverse lasting effects from reverse sneezing although it may sound and look unusual.
Ongoing sneezing can be a symptom of several underlying conditions. If your dog has persistent or worsening sneezing, other respiratory difficulties, or bloody or yellow discharge consult with your veterinarian as your dog may need treatment or medication.