There are two areas to look at.
A common failure point is the flex joint in the steering column. Look right below the front suspension tower and you'll see the joint. Inside that joint is a flex/braid that passes the earth connection. If it breaks, the horn won't sound.
The second point is a gaggle of black wires that connect to the steering rack. If that connection point is corroded or the bolt has come out, you may loose the earth connection for the rack.