If only one of the sensors has a green light glowing then either the sensors are dirty or they are not aligned properly. You need to move them around until the both sensors have lights glowing. If this does not happen then one of the sensors are bad - you then need to go to a garage door place and request a new pair. Best bet is to bring one of the sensors with you.
Sometimes, the circuit board may be bad as well. But I would try:
1. Cleaning the sensors
2. Making sure they are aligned
3. If #1 and #2 do not work - change the pair (they are only sold in pairs)