Baby hummingbirds are not good fliers and sometimes they will fall out of the nest. If the nest is low enough, you can put it back. If you cannot reach it, make a makeshift nest out of a box or small container with cloth to pad the bottom. Attach the surrogate nest to a limb as close to view from the original nest as you can get. Do not put food or water in the nest. Do not leave the bird on the ground, as it will become prey for a night feeder.
You can do the same for a nest that has been blown out of a tree. Make the surrogate nest and attach it as close to the original nest as possible. The mother will find it and continue caring for her young. Remember, these birds mature quickly and your nest only has to last a week or two.
Do not handle the baby bird any more than necessary, and remember that the bird is very fragile.
The picture is of a makeshift nest made out of the bottom of a plastic bottle by CaSondra of Boise, Idaho. She reported that the mother continued to feed the babies and they fledged.