If your dishwasher won't drain, you may have a blocked drain. However, in many cases, you can find the issue on your own without the help of a plumber. Here's what you need to do:
1. Run a anti-blockage solution down your dishwasher.
Before you start trying to find the clog, try to eliminate it with a homemade anti-clogging solution. Mix together baking soda and vinegar and pour some into your empty dishwasher. Then, run the dishwasher. When you are done, check to see if the water has drained. If not, try one of these following two steps.
2. Check the pipe leading away from the dishwasher for blockages.
In most cases, you should easily be able to see the pipe that drains your dishwasher. If you have a garbage disposal, the pipe usually leads from the dishwasher to the disposal, but if you don't have a disposal, the pipe simply leads to the rest of the pipes that exit your house.
Look under your sink and find the pipe that drains your dishwasher. Turn off the water, disconnect this pipe and check it for blockages. If you see any blockages, push them out of the pipe carefully using something long like a metal hanger or a small dowel. That works for sturdy drain pipes, but if you have a flexible drain pipe leading out of your dishwasher, you won't be able to get anything inside it. To remove the blockage, hold the end of a garden hose next to the pipe, turn on the water and let the force of the water push out the blockage. In lieu of that, consider replacing this pipe.
3. Clean the filter of your dishwasher.
If you don't find any blockages in the pipework, the issue may be in the filter of the drain of your dishwasher. To access this area, you may need to take apart a significant part of the dishwasher. To get started, remove the trays from the dishwasher. Then, grab a towel and lay it down in the bottom of the dishwasher -- as you take out screws so you can remove the spraying arms of the dishwasher, the towel will help to make sure that none of the screws fall down your drain.
Then, piece by piece, remove all the parts of the dishwasher leading to the drain. In most cases, you will remove a drain cover and a motor cover until you find a spinning blade and a fine mesh filter. At that point, carefully check the blade for debris and clean it. Then, remove the filter and thoroughly clean that. Even little things on the filter can block the water from going down the drain. As you put together the dishwasher back together, make sure that you clean every part.
If your dishwasher still doesn't drain, there may be a blockage somewhere else in your system. Contact a plumber to help you find and remove it.