It’s best to leave the baby bird alone and try to ensure its safety. The parents are most likely caring for it, whether it is immediately evident or not. If you can gently and easily place the baby in a more hidden location (e.g. under a bush, in thick ground cover), do so. Do not try to put it back in the nest; it possibly fell out once already (or it fledged and was on its first flight). Most of all, keep your eyes out for wandering cats/other predators, including dogs and children that might accidentally harm the baby bird.
Do not try to feed the bird or give it water. This can cause more harm to the bird. There’s more information in our blog on this subject.