One possible reason a battery charger may stop working is if the charging port is damaged. If the port becomes bent or broken, it may cause the charger to no longer make a proper connection with the device. Another possibility is that the charging cord itself is damaged. If it becomes frayed or otherwise damaged, it may not be able to transfer power properly from the outlet to the device.
Another potential issue could be with the actual battery itself. If the battery is old or has been used for a long time, it may no longer hold a charge as well as it once did. This can cause problems with chargers because they are designed to work with batteries of a certain capacity. When that capacity is no longer there, it can prevent the charger from working correctly.
Finally, there could be an issue with how the charger is being used. For example, if you're trying to charge a device faster than what the charger is rated for, it could damage both the charger and the device. Always follow manufacturer's instructions when using any type of electronic equipment!