A couple of problems could keep a dishwasher from filling with water. One of the most common issues is a stuck or defective float switch.
The float switch is part of a usually round-shaped assembly located on the bottom of the dishwasher, most likely located close to the door. This switch keeps water from over-filling your dishwasher. First, check to make sure nothing is caught under the assembly that would prop it up. If it is clear, check to see if the float assembly moves easily up and down. If it does, the float switch may need to be repaired or replaced.
Another issue that keeps your dishwasher from filling involves water lines. If all of the faucets are working in your home and your dishwasher isn't filling, you may have a problem with the water supply valves or water inlet valves. These pieces can get clogged with lime scale deposits, especially if you live in an area with hard water.
Either of the above cases are issues that can you can fix yourself at home. Replacement float switch and float assemblies, as well as new valves, can be ordered from your dishwasher's manufacturer. Before beginning work, take appropriate safety precautions and be sure that both the water and the electricity are turned off.