There could be a few reasons why the button stopped working, particularly for older chargers - or chargers that have been unused for a longer period. The contact surfaces of the RESET button may have oxidized and become stuck.
Please try the following:
Connect the charger to the mains.
Power on the charger but leave disconnected from a battery, periodically pressing the RESET button as this may help remove any oxidation.
Also try pressing the RESET button a litle firmer and repeat several times.
If the issue persists, the button is most likely defective.