First check the drain line where it connect to the sink, make sure it is not clogged there, also check if the pump is working, if both things are ok then the problem is the--piston & nut.
The piston & nut is a litttle plastic thing with a spring on it this is located on the inside of the dishwasher at the back this is how you get to it,
at the back of the dishwasher on the inside there is a strip of plastic with mesh on it this is held in place with four 1/4 inch screws use a 1/4 nut driver to remove them, lift up and remove the strip, under the strip you will see the piston & nut sometimes you can use your fingers to un screw it if not use a pliers to move it then use your fingers to un screw it, take it to your local parts store or order it from --appliance parts pro--.