would be best to check it's not the horn first - use a couple of test leads to connect it directly to the battery. if it works, the problem's with the wiring/'button'
i can't remember how many connections the horn has - if it has two, they'll be live&earth; if it has one, it'll be live and the horn will be earthed internally through its mounting to the shell (take it off and clean mounting points if they're corroded)
you could test the feed lead/s as well by holding them against a spare bulb whilst someone pushes horn
basically, find out if it's the horn or the wiring first