A dead sunflower is jsut that! They are tough, and if you manage to kill one by not giving it water in hot weather there is little chance you can bring it back to life. If it has fungal problems, you can apply a registered fungicide and you may revive the plant. I personally just pull them out if they get really weak. Try the next season again and make sure to mulch your seedlings, choose rust resistant seeds and make sure not to get the leaves wet when you irrigate. If need be, apply a fungicide as per the advice from your local agricultural extension officer.