Is it mounted under the front bumper, and have you been though a deep puddle, it may have got full of water. Try cleaning the connectors on the horn. If you have a voltmeter see if you have voltage across the connectors when te horn is pressed. I have been told hitting it hard might make it work. If you meed a new horn and you cannot get a second hand one or a reasonably prices actual spare, then one from most German cars will probably fit, you hight have to rewire the connector and make the bracket fit but most horns are the same.