MMH  >  Exterior Doors  >  Garage door

When I try to close my garage door it only goes a little distance then reverses - what is wrong with the garage door opener?

I have a Craftsman garage door opener. With the garage door at the open position, I push the transmitter to close, it goes down approximately 4 - 6 inches, then reverses back to fully open.

Why does my garage door close, then open?