Test the horn itself by running 12 volts directly to it with a jumper wire. If silence, replace horn. If honk, start checking continuity of wiring to find the short. If the horn hasn't worked since you got the car, or if it quit after a wreck or for example a turn signal combo switch replacement... closely inspect the wire behind the driver's airbag. The tiny stud breaks off that black plastic tube if a guy gets too rough with it. That will make the horn not work because no switch contact.