Diagnosis
A clog in the drain hose or air gap, or a kinked hose. The pump runs but can't push the water out.
How to Fix It
First, check the drain hose under the sink. Disconnect it from the disposal or drain standpipe. Blow through it to clear any blockage. Ensure the hose has a high loop (secured to the underside of the countertop) to prevent sink water from back-siphoning.
If That Didn't Work
Check the dishwasher's internal drain line and pump housing for a clog, as described in the grinding noise fix. Also, inspect the check valve in the drain pump assembly. If it's stuck open, water can flow back into the tub.
Heads Up
Never use drain cleaning chemicals like Drano in the dishwasher. They will destroy rubber seals and components. Use only mechanical cleaning methods.