While a dishwasher failing to drain may be a result of a broken pump or an obstructed house drain line, such an issue is usually caused by a clogged sump. Clearing the sump involves pulling the racks out, removing the sump cover and pulling the source of the blockage out.
To repair a dishwasher that's not draining due to a blockage in the sump, start by removing the racks then rotate the spray arm gently until it snaps free. Most dishwashers also have a nut holding the assembly together. Turn the nut until it snaps free, and lift off the assembly frame.
If there is a screw holding the water hose in place, remove it. Gently pull on the connector holding the hose pieces together until it snaps free. Rotate the hose assembly to the left, and move the hose out of the way. This should expose a drain with a cover. This is the sump.
Wiggle the sump cover until it comes free. Reach inside, and take hold of the source of the blockage. There may be a lot of food and debris stuck in the pump, so keep clearing it out until nothing is left. If necessary, use a blunt instrument such as a craft stick to loosen any debris. Reverse the above steps to reassemble the sump cover, hose and spray arm.