kenmore microwave starts as soon as you shut door

My microwave starts as soon as you shut the door, you can hit start or cancel and it just continues to run. the light comes on, but the turntable does not turn, and you can hear the fan running. you just cant get it to turn off. All the controls still seem to allow you to do things, I even put food in there to cook, but it does not cook, just makes a noise like its running. Can you help?