The side with the red light is usually the sender. If the other side does not have a green light on it, then the infrared light from the sender is not reaching the receiver, the receiver is bad or the wiring is bad.
I would start by just visually verifying if the sender and the receiver are pointing at each other. If they are not, say, either of the two got an accidental slight kick and is now misaligned, restore the alignment and observe the green light. Another trick is to just shine an infrared signal from any remove control in the house into the receiver and see if the light comes one. Simply press any button on any remote, hold it and point it onto the receiver. If there's still no light - the wiring from the safety beam receiver to the motor may be bad or, indeed, the receiver itself.
The receiver and sender often come with only 12" or so short stub of cable which will have to be spliced to a cable going up to the motor. That splice (crimp or wire nut connection)Â may go bad if it wasn't properly done. The receiver gets about 6V from the motor. If you have a multimeter handy, you can check whether it's getting the voltage it needs. If not - check the wiring.
If it does get voltage yet still does not work, replace the sender-receiver pair. They are not very expensive , $15 buys a nice pair: