Belt, Water Pump
Although few dishwashers have a water pump belt, if the dishwasher won't drain there might be a worn or broken water pump belt. After removing the lower kick panel the motor can be seen in the front, the water pump belt is on the bottom of the motor.
If the dishwasher won't drain, the drain pump might be defective. Test the pump motor with an Ohm meter. Drain pumps are not repairable, if it's defective it will need to be replaced.
The drain impeller is attached to the pump and pushes the water out of the dishwasher, through the drain tube and into the garbage disposer or drain line. If the dishwasher won't drain, the drain impeller might be broken or worn out. Replace both the impeller and seal at the same time.
Drain Solenoid Kit
If the dishwasher won't drain, the drain solenoid kit might need to be replaced. The drain solenoid is an actuator that opens a flap to allow the water to drain.
The drain valve can get clogged with debris and should be cleaned out if the dishwasher won't drain. If it seems clear of debris but won't actuate, it will need to be replaced. The drain valve is not repairable.
Pump and Motor Assembly
If the dishwasher won't drain, the pump and motor assembly might be defective. If the motor runs during the drain cycle, check for a blockage in the drain line. If the motor won't run during the drain cycle or if it just hums when it should be running, the pump and motor assembly might need to be replaced.
Other Causes and Conditions
Clogged or Improperly Installed Garbage Disposer
If the dishwasher won't drain and the garbage disposer was installed recently, the installer might have forgotten to knock out the drain plug from this disposer. All garbage disposers are sold with a solid plug in the drain tube for situations where a dishwasher isn't connected. However, if the dishwasher is connected, the plug will need to be removed. If the disposer wasn't replaced recently, remove the dishwasher drain hose from the disposer and clean out both the fitting on the disposer and the end of the drain hose. These are the most common places to find an obstruction if the dishwasher won't drain.
If the dishwasher won't drain the drain hose might be clogged. Remove the drain hose from both the dishwasher and the sink drain to clean it out. The most common place for an obstruction is at the garbage disposer.
If the dishwasher won't drain the check valve might be stuck closed. The check valve is a device which prevents water from returning to the dishwasher after it has drained. If the valve gets stuck closed, it will prevent the water from draining. The check valve is located in or at the outlet of the pump that the drain hose attaches to. Sometimes the check valve is accessed from the inside of the unit depending on the model.
If the dishwasher won't drain the check ball might be stuck. The check ball is a small plastic ball in the check valve which prevents water from returning to the dishwasher after it has drained. If the ball gets stuck, it will prevent the water from draining also. The check valve is the device that the drain hose is attached to on the dishwasher.
Those are the main things to check. You might find some additional ideas on the Dishwasher Not Draining page. Good luck!