Find the horn.
Disconnect wire to horn.
Install test light or volt meter between horn wire and ground.
Have an assistant try to honk the horn from the steering wheel while you see if there is voltage to the connector.
If you have voltage when trying to honk the horn, replace the horn. Look at how it is mounted to see if water was getting into the horn opening, if so, figure yout how to mount it in a better protected maner.
If you don't have voltage, you will need help in understanding the circuit so that you can better troubleshoot it, but usually it is a bad horn.
