I went to supermarket recently. Locked car to go in but noticed the driver door was not locked. Didn't want to hang around so we did shopping and returned to car. However, the key did not work at all. Shows a red light and that is all. Walked home, leaving wife in car and returned with spare key. All works fine with that key. Changed battery on fob and still does not work. 2013 C220D. Any ideas please?