DIAGNOSIS A clog in the drain hose or household plumbing, or a stuck drain pump.
FIX First, check and clean the sink-side air gap or garbage disposal connection. If you have a disposal, remove the drain hose plug. Then, disconnect the dishwasher drain hose from under the sink and blow through it to clear any obstruction. Also, check the drain hose for high loops or kinks under the dishwasher.
IF THAT DIDN'T WORK The drain pump might be jammed. Unplug the machine. Access the drain pump (usually behind the lower kick plate). Manually check the pump impeller for rotation and clear any debris. A failed pump won't make its characteristic humming noise during the drain portion of the cycle.
HEADS UP Never use drain cleaner chemicals in the dishwasher. They can destroy rubber seals and the plastic tub. Use only manual clearing methods or a baking soda/vinegar solution.