I had the same problem with my horn, it would go off at random, especially when it went past nice looking girls HEHE,
But seriously though, mine did have the problem and found that one of the wires had split exposing the wiring and would touch the metal plating. This would spark and trigger the horn.
Chances are that its a blown fuse or maybe its burnt through the wire?
Its a 15A fuse and it says HAZ-HORN and its in the engine bay.