There could be a number of reasons for the spray arm is not spraying. You do not indicate if the arm is spinning or if the unit is pumping out water into the tub. The first thing I would recommend is checking the holes on the spray arm to see if they are clogged with calcium deposits and cleaning them out. If the dishwasher has plenty of water in the tub & is not spraying water out of the spray arm, then I suspect the wash motor impeller or pump assembly.
If you can hear the motor running but there is no water coming from the spray arm, the dishwasher pump could possibly be starving for water. Check for a dirty screen below the lower spray arm. Most likely the dishwasher is not filling with enough water due to a water supply restriction.
Try this: Manually add about a quart of water directly into the bottom of the tub after it quits spraying. If it begins and continues to spray water then the water supply to the dishwasher is restricted. The water inlet valve on the dishwasher has a filter screen which could be clogged up or it could be a restriction in the water supply to the water inlet valve.
I recommend resubmitting your question with additional details if needed for further assistance.