A car horn is broken for, generally, one of three reasons; and can consequently be fixed in one of three ways.
If your car horn is still making a sound, but it is very quiet, then 1 or more of your horns have stopped working (most cars have multiple horns). To fix this:
1. Pop the hood and locate the horn(s) behind the cars grille on the radiator core support.
2. Remove the wire connector (press down on the lower end and then pull the wiring out).
3. Remove the spade lugs and mounting bolt, clean all components, and reattach.
4. If this cleaning does not work, purchase a replacement horn.
If your car horn is making no sound, you may have a fuse problem. To fix this:
1. Read your owner’s manual to locate the fuse box, as well as the horn fuse, in your car.
2. Remove the horn fuse with a pair of pliers.
3. If the metal strip inside is broken, then your fuse is broken and needs replaced.
4. Purchase replacement fuses at an auto supply store.
5. Install new fuse and try the horn again.
If your horn still is not working (or your fuses were not broken), check to see if your check airbag light is illuminated on your dashboard. To fix this:
1. If your check airbag light is on, then it is highly likely that your airbag has expanded and could be interfering with the clock spring component that allows power to reach your horn.
2. Unfortunately, the only option for this situation is to take the car to the mechanic.
3. The air bag will need to be removed and properly installed again, so that power can reach your horn, and your airbag can deploy properly if it is ever needed in the case of an accident.
Sources: How to Do Anything