Usually an egg laid underwater or above ground is infertile (I don't know how the turtles know this, instinct I suppose) but its possible that it was fertile if she had no other options...if keeping male & female turtles together, always provide an accessible spot for eggs! Some turtles will refuse to lay an egg in a bad spot, & they will DIE from this!
Also if a female is bred too often, it can make her sick & weak enough to kill her.
Is the container the egg is in filled with substrate to keep it moist? If its dried out then that egg is probably now dead, I'm sorry to say.
The best place for box turtle info is boxturtlesite.info
- the section on box turtle care A-Z. There is a section on breeding & on how to hatch eggs & care for hatchlings that will really help you.