If your vehicle is showing horn doesn't work at all when pressing the steering wheel, here are the most likely causes and the parts you'll need to fix each one.
The fuse protecting the horn circuit has blown, cutting off power. This is often caused by an aging horn drawing too much amperage or a short in the wiring.
The horn itself is mounted behind the front grille, exposing it to rain, road salt, and debris. Over time, water intrusion and corrosion cause the internal diaphragm to seize.
The relay that acts as a heavy-duty switch to send battery power to the horn has burned out or developed pitted contacts.
The clockspring is a ribbon cable inside the steering column that maintains electrical connection while the wheel turns. It commonly breaks from age or if the steering wheel is allowed to spin freely during steering rack repairs.
The electrical connector at the horn may have been left unplugged after recent front-end bodywork, or the horn's mounting bracket has become too rusty to provide a proper electrical ground.
In modern vehicles, the steering wheel switch doesn't directly power the horn; it sends a request to the BCM. A failing BCM or corrupted software can cause it to ignore the horn request.