Remove the horn. Get two wires and connect them to the two lugs on the horn. Put one wire on your hot battery terminal and the other on your ground. Does it blow?
If no, then try adjusting the small flat head screw on the back. If it still doens't blow, replace the horn.
If yes, then put the horn back where it came from.
Next, if your horn blew, chase the black wire and make sure it is securely connected. This is the ground. The hot wire is brown. It comes from your horn button. When the button is pushed, power goes through the button to the horn. This means that the button ALWAYS has switched hot power on the back side of the button. Take a multimeter and turn on your key and see if you have voltage there (you will have to open up your left switchgear). If you have no power, the brown wire isn't connected to your wiring harness which has a number of brown wires that will be powered, trace the wire back to the harness and fix the break. If the wire IS powered, check the other side of the button for power.
OK, the brown wire that hooks to the horn... connect your multimeter POS lead and put the NEG lead on a frame ground. Put the meter in VDC scale and turn the key on. Then push the horn button. If you have power, the horn will work (unless ground is bad since you tested horn) and if you don't have power, the horn button is the problem. Clean the contacts with an emory board or steel wool or replace.