KitchenAid dishwasher not washing properly

I have an old KitchenAid dishwasher that has been very reliable. However it has suddenly stopped working. Everything appears to be OK but the washer spray arm does not seem to turn, the soap dispenser is not being completely emptied, and nothing is getting washed. The arm turns freely and does not appear to be clogged. I have checked the motor and impeller and they are OK. The machine is getting water and seems to be filling but I am not sure if the water level is sufficient. The machine progresses through the wash cycles OK and drains satisfactorily. Any suggestions as to what might be the cause and what I can do to fix it would be appreciated.