Contrary to popular belief, birds do not abandon their young just because a human touches them. If you are safely able to reach the nest from which a baby bird has fallen, it is totally fine to put it back.
However, keep in mind that not all grounded birds are in peril. If you see a baby bird with a fully-feathered body and a short tail feather hopping around the ground unable to fly, it may be a fledgling. In that case, leave it alone. Since birds learn to fly from the ground up, adolescent avians generally remain on the ground for a few days before finally taking flight. In the meantime, the best help you can give the fledgling is to keep cats and dogs away from the area.
Of course, if the baby bird appears injured or in imminent danger, it is best to contact a licensed wildlife rehabilitator.