Make sure the sensors are properly aligned. If they are each one will have a small light that is lit. You could also have a bad sensor.

I have a Sears Craftman Garage Door opener that was installed prior to me purchasing the home, 20 years ago. The door suddenly stopped working so I have to manually open it up and down. When I engage the remote the motor comes on but nothing happens.

I can see the a black cylinder spinning but for some reason the gears(white plastic spindles with teeth) do not become engaged.

Thanks in anticipation of your response.

jassyjaz,

It sounds like that main drive gear is stripped out on you. You would have noticed white saw dust looking material on the cover and some probably fell out when you took the cover off.