Fawns are often left alone while their mothers feed. She does this to keep predators away from the fawn. Due to the spotting of the fawn's fur, it blends right into its surroundings and won't be seen. If you come across a fawn that is not crying or is not injured, leave it there and check back 24 hours later.
On the other hand if the deer is crying and/or injured please contact your local wildlife rehabilitator as soon as possible.