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If a dishwasher sits unused, for a period of time, the seals in the motor and pump can become frozen together. If you have high minerals in your water, it can stick the seals together, and the motor cannot turn on; often the dishwasher will fill with water and just sit there "humming".
Usually this is fairly easy to repair and to get running again. Disconnect power; remove access panels and reach in with your hand, to turn the cooling fan blade and break it free. Reinstall power and try the dishwasher. If the "sticking" occurs again, the motor and pump assembly will probably need to be replaced.
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