It’s best 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 (versus fledged and was on its first flight) once already and lived through the experience. Most of all keep your eyes out for wandering cats/other “predators” (including dogs and children that might accidentally harm the baby bird).