The receiving unit could be part of the manual switch on the wall (easy fix) or inside the motor housing (not so easy). Look for an antenna.
Depending on the age and/or make of the opener, you should be able to get just the receiving unit. You’ll probably have to go straight to the manufacturer.
If you’ve still got it, dig out the instruction book and look for a part number.
Alternatively, opening the unit and looking for loose or corroded connections, broken wires or such might get you somewhere.
However, assuming you can’t repair it, you’ll then need to put it back together and reinstall it. Otherwise, you’ll have no opener at all till you get a new unit (which is a major pain to someone as lazy as myself). Plus you’ll have to track down the key to the manual lock!!
Ah!! The continued joys of home ownership!
Edited to add: Opps! We were both typing at the same time!!