While I was going to school in the '70s in our home economics class we got to watch documentaries on drugs and alcohol, birth control and family planning and problems in child behaviour. One of the problems was waking up at night. The professionals suggested comforting the child in a calm and loving manner, turning on the light to make certain the child feels safe and it can see that there's nothing amiss and finally putting it back in bed. If the child still has a problem, then it would be wise to have a short story ready to read to him/her.
After that you must leave in a manner that the child understands this is it; time to sleep. By the way when the family atmosphere is for some reason hectic or tensed, the child will probably be so too. Make sure that the home environment is calm for the child in order to avoid fear and insecurity from festering inside him/her.