Sounds like the down limit is out of adjustment. Does the opener rail bow up when the door hits the ground? If so back the down limit off a turn or two and try it. If your are standing in the garage looking out the limits will be on the left hand side of the opener. The down limit is the limit closest to the door.

If your limits are ok turn your down force up a quarter of a turn and try it.

If adjusting the down limit control doesn't fix the problem, remove the cover (8 screws) and check the wires by the up and down limit controls. Because the center contact moves back and forth, it might have fatigued and broken, so there is an open circuit for the shutoff and it will not work. Reconnect the broken wire.