Maybe the horn itself has failed? Not sure if the horn is easily accessible but if so, check if power is getting to the horn using a test light or VOM.
Red, I don't think this is just a key fob issue. The horn does not work even when the horn button is pushed on the steering wheel.Try this: on the key fob, press the lock and unlock buttons together until the car turn signals turn on.
Basically you can program nissans to have the horn come on or not when you press the key fob.