I've worked on many dishwashers and disposers and the most common problem is where the drain hose fits to the disposer. Take the hose off there,Usually held with just a hose clamp , and see if the hose or the inlet to the disposer is clogged. Be gentle but firm with getting the hose off because the inlet is old and brittle. If it breaks off you might have to install a new disposer. If this isn't the reason, check the other items that Steve said. If you find the water pump is bad, install a new dishwasher. P.S. Don't forget to reconnect the drain hose you took off. Ron J.