If there were any shells around the open hole, a predator likely got them. It's also possible that the turtle was scared by your presence and abandoned the nest she was digging for a better spot. (They often will "test" places and dig in a few of them before deciding on where to actually dig.)
As for the covered nest, if it's possible to do so, you could protect it by covering it with a square of something like hardware cloth and staking the corners down (I don't know if you can do this if the land isn't yours and it's frequently mowed, though). You'd have to check on it periodically and let out the hatchlings when they hatch, though. Four summers ago I protected a nest that was dug on our front lawn with a wire basket and 5 painted hatchling came out of it.

There are other predators besides crows---if you have any raccoons where you are, they're very good at locating eggs and digging up nests.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-