What could have happened if I hadn't taken my dog to the vet?

Death. If her blood pressure and heart rate were that low then she wouldn't have made it much longer. Why did you wait so long to take her in the first place? Tranquilizers and sedatives are always very dangerous and potentially deadly in the case of an overdose and you should have brought your dog (and the reserpine) to the vet for monitoring as soon as you realized that she had eaten it.