What does it mean if an automatic blood pressure cuff will not register your blood pressure?

Arrhythmia. It could mean that you are having an arrhythmia -- because the automated home BP machine is designed with a normal/regular rate and rhythm most commonly seen -.
Buy a new one. Most of the time that is because the cuff is not working properly. If your blood pressure were to low for it to register, you probably would not be around to know that. I would replace the batteries and if it still does not register, try taking someone elses blood pressure with it.