Observe the nest for an hour or more, from a distance, to ensure the parents have heard and answer the baby's feeding chirps. They often fall, jump, are pushed or predated. The parents will feed them even if they aren't in the nest. They can be placed back on a nearby branch and then left alone. It is critical that this is done as soon after the baby is found as possible.
Fledglings are ready to leave the nest.
This is a natural event, and it doesn't matter how many times you put them in the nest, they will immediately jump out. They don't fly well for the first day or two. They can be placed in the nearby bushes for cover, and the parents will feed them, and show them how to hunt for food, and how to recognize predators - including humans. Ask neighbors to keep dogs on a leash for a day or two, and keep cats inside - the babies will soon learn to fly well enough to escape.