BetterThanOne saidThanks for the advice.
I managed to get it back in the nest so I'll have to see what happens. It was still alive and active. I noticed the baby birds about a week ago so they are still very young. I guess it is a good thing that I live on the third floor of my apartment complex so nothing could get to it as easily.
The parent(s) may have pushed it out, or its siblings. Or just a strong wind gust might have swung the nest if it's on a branch, catapulting the chick out. You did the right thing, and either the parent(s) will resume feeding it, or not.
A curious aside: certain species of the cuckoo are called parasitic brooders. They lay an egg among the eggs of another unrelated bird. That egg usually hatches before the others, and the chick is fed by the other parent. In a short time, the cuckoo chick begins to eject any unhatched eggs from the nest, and also any other non-cuckoo hatched chicks.
There are only 3 such cuckoo species in the US, so the chances of that in this case are low. But if you see the number of chicks declining in the nest until there is only 1, and it doesn't resemble the parents, then it may be a parasitic brooder cuckoo.