The dishwasher typically drains into the kitchen sink drain or a garbage disposer. If the dishwasher drains into a garbage disposer, run the disposer to clear the drain system. Do not put your hands into the garbage disposer. Serious injury could result. You should always turn on your garbage disposer prior to running the dishwasher to clean out any food or debris within the drain system.
Check drain hose where it connects below the sink for blockage or a kink in the hose.
New dishwashers come with a white corrugated drain hose, which should be used. Re-using old, worn out drain hoses may promote kinking and blockage. Check the air gap if one is installed. It could be clogged. Some state or local plumbing codes require the use of a drain air gap between the built-in dishwasher and the drain system of the home.