Why does your garage door transmitter not work?

The first thing to do is to change the battery in the transmitter for a brand new one. If it still doesn't work then, if you don't have any skills in diagnosing electrical faults - which may be true because you needed to ask this question here - the best advice anyone should give you is to call a skilled fitter of motor-operated garage doors, or a licensed electrician, to advise you.