Fix garage door opener sensors by looking for any physical objects that are preventing the sensors from seeing each other. This includes any dirt or other debris that is on the lens. Depending on the model of sensor, the lights may indicate whether the glass eyes of the sensors are obscured.
Make sure the unit is powered on and that the sensors are getting electricity. If the lights on the sensors aren't on, check the power to the main part of the unit.
Open up the sensor units themselves. Confirm that the wiring is soldered to the proper places, that there are no foreign objects inside them and that they are dry inside.