The Nexus Crystal is a beacon between our world and the world of Mystical Dragons. They need this crystal to survive and thrive in our world. Without it they will not grow or breed.
Before you rez or birth any dragons always make sure you have the latest Nexus already on your land. If it turns red it means you already have one out so pick it up and find your other Nexus, pick that one up too and then re-rez the newest one.
If your dragons are showing there is no Nexus and you just rezed the new one, wait about 45 minutes before panicing, it should update them during that time. If that still doesn't work, you can get advise in the Mystical Creatures group or open a ticket if it's an emergency situation.