Since you did not include the model number off of your dishwasher I am only able to provide you with some basic information.
Dishwasher will not drain due to;
If a cycle has completed and there is standing water in the bottom of the tub. Check to make sure the cycle is finished.
If a dishwasher is creating too many suds, the drain pump will pump out the water, but will not be able to drain the suds out.
Excessive suds conditions are caused by multiple things, mostly using too much Detergent or Rinse Aid.
If the Air Gap is clogged it will affect the ability of the dishwasher to drain.
The Air Gap usually has a chrome cover and is located on top of the sink or countertop near the dishwasher.
Some water in the bottom of the tub around the pump module is normal. This is to prevent the pump seals from dry rotting. Some dishwashers run the drain pump at the beginning of the cycle to drain the remaining water that may have been left in the unit or collected while loading dishes.
Water should not completely cover the top of the pump module / filter screen. If more than 1" of water is present, there may be a draining issue.
A drain hose is most likely to clog is where it connects to the sink drain, house drain, garbage disposal or air gap.
To check for a clogged Drain Hose
Materials Needed: Old towels, to mop up any spilled water, a screw driver or pliers, to remove the hose clamps and a bright light source, to inspect the drain hose thoroughly.
Loosen the hose clamp where the hose is being checked.
Remove the drain hose from the connection.
Check the drain port on the sink, disposal or air gap.
Check the drain hose opening for blockage.
For more details, reply to this thread.