When mine stopped working I went out and got a new one. On a Mk2, it goes at the front of the engine bay, near the drivers side (for a RHD car!) headlamp, Mk1 could be different. If the wiring is all okay (use a multimeter to check you get 12v or so across the two terminals when the horn is pressed), then grab a replacement horn either from Halfords or ebay etc...
Punto Mk2 ELX 8v in Teseo Blue with the electric Sunroof and CD changer, doing 50mpg whilst passing down country lanes