If you no longer hear water moving in your AeroGarden, then the water pump inside the water bowl has stopped working. Before calling AeroGrow Customer Service for a replacement, please take a minute or two to perform a couple of simple troubleshooting steps to determine whether the pump stopped working only because it's not getting electricity.
Your pump has a normal "off" cycle (it is not programmed to be "on" all the time for most plant selections). To check to see if it's OK but just “off”, push and hold the "Select" button on the base for about six seconds to start the AeroGarden's self-diagnostic cycle during which all of its systems will go on and off: the grow lights, the plant selection lights, the "Add Nutrient" light, the "Water Level Low" light, and the water pump. During this procedure, if you hear a faint hum and the sound of water gurgling, then your pump is fine - just in an "off" cycle on your plant selection.
If you do not hear the pump come on, continue with the next troubleshooting step: cleaning the electrical contacts that supply electricity to the pump.