Feeding ducks cracked corn and nothing else is like feeding your kids candy and nothing else. They might survive to adulthood, but they're not likely to do well. Corn is basically sugar and not much else. Quick as you can, get down to your feed & seed store and buy some chicken layer pellets. You will probably have to take the corn away altogether--they are not likely to want to switch over, any more than a kid who was raised on candy would want to switch to green beans and roast pork. But get them hungry enough and they'll do it eventually.
They may never lay. Malnourishment can cause permanent damage. But you might get lucky and the improved diet may cause them to start laying again. I sure hope so!
And don't beat yourself up--we all make mistakes. That's what we come here for--so we can correct each other's mistakes!
Your ducks will be much happier & healthier with your new knowledge (once they get over withdrawal from the corn!!).