Sounds like one of the two sensors (located near the bottom of each door rail and close to floor), sometimes called trip sensor.
1. Make sure no objects are blocking the beam
2. check all wires (looks like telephone wire) attached to the trip sensors.
3. check same wires inside the door opener housing
4. if all wires are attached and none are loose. do the next
5. disconnect one of the wires on each sensor and connect one of the two unused wires – plus do the same color wire on the door opener. (total of 3 locations)
6. if still not working disconnect the other wire and connect the fourth (second unused wire) in all three places.
Good Luck.