One thing to check is the metal leaf spring on the indicator switch that comes into contact with a metal ring on the wheel. These often get gummed up with all kinds of crap and prevent the horn from working, and sometimes they fall off/break.
Look at the bottom of the steering column between the wheel and ignition shroud, you should be able to see it there. get something pointy and clean out the crap stuck there and give it another go!
Also try removing the centre cover for the steering wheel and make sure both wires are connected to the push horn, and also try touching them together when the ignition is on to see if its your push horn that is the problem.