The most likely reasons a car horn fails to work correctly are that a fuse is blown, the components of the horn have gotten dirty, or there is a problem with the clock spring. When repairing a car horn, start troubleshooting from the first cause, as it is the easiest one to fix.
Use the owner's manual for the car to find out the location of the fuse panel and which fuse is responsible for the horn. Gently pull out the fuse using pliers, and inspect it. If a metal strip in the middle of the fuse is broken, the fuse is blown. Replace the fuse with a new one.
If the fuse isn't blown, locate the horn under the hood of the car, remove the connectors off the horn one at a time, and test the horn. If it still doesn't work, take out the spade lug with a lug wrench, and remove the mounting bolt using pliers. Place the components in parts cleaner for about 10 minutes, clean them, and put them back in place. If that didn't help, install a replacement horn in the vehicle.
If the horn doesn't work, but the fuse isn't blown and the dash light for the air bag is lit, there may be a problem with the horn's clock spring. To repair it, have a mechanic check to see if the air bag is blocking the clock spring.