Images would be very helpful here if you can upload some. I suspect that your plants may be suffering from black rot, a common disease of grapes which is caused by a fungus. It causes infected grapes to turn black, shrivel and mummify. The disease can also affect the leaves, appearing as brown, circular lesions. Control of the disease is usually achieved through fungicide application and also cultural control methods. You will want to make sure that all the mummified fruits are removed from the vines at the end of the season, along with any that have fallen to the ground. Please add some images to your question so that we can try and get a definitive diagnosis.