find the horn and check the connector on it is not corroded, this is usually the problem. then use a multimeter to check the earth is actually earthed (if it isnt find out why), also check there is a live signal getting to the horn when the horn button on the steering wheel is pressed. if there isnt you should remove the airbag (disconnect batt for 15mins first) and check the horn switch looks ok and has continuity when pressed, and a live signal going to it. there will also be a fuse in the fusebox, check the diagram on the back of the fusebox cover to check which one it is, i would check the fuse first.