First, shut the power to the opener off. Sometimes this is done by pulling a fuse in your garage or by flipping a switch on an electric motor opener. Then, check the cable to see if there are any visible breaks in it. If the cable is broken or stretched, you will need to replace it. If there are no visible breaks in the cable, proceed to the next step. If only one end of the cable is broken or stretched, you will need to open up the motor housing and replace that one end. If both ends are broken or stretched, you will need to replace the whole cable.