Few things can match the annoyed frustration you feel when your car horn suddenly starts blasting and won't stop. It's a situation that cries out for an immediate solution. The first thing to do is to not to panic. Keep your cool and approach the problem in a methodical manner. Getting the horn to stop may be as simple as tapping the horn switch on the steering wheel a couple of time. If this doesn't work, there are other options to consider as well.
Press or tap on the horn switch on the steeling wheel. The horn switch in the steering wheel may have become stuck. Tapping or pressing the horn switch may work to free it and stop the horn from blowing.
Turn off the engine and locate the fuse box if tapping the steering wheel didn't help. Look carefully at the chart for the fuses on the box and try to locate the fuse for the horn. Pull out the fuse for the horn.
Open the bonnet and locate the horn if pulling the fuse didn't stop the horn. The rear of the horn may be visible. Disconnect the wires on the rear of the horn. Cut the wires if it is not possible to disconnect them. Tape the two ends of the wire with electrical tape to prevent them from causing a short.
Locate the main wire that connects the horn to the car battery if the preceding steps don't work or you can't locate the horn. Disconnect the battery cables, beginning with the black (ground) cable--be sure to set it aside where it can't touch metal--followed by the positive cable. With luck this will reset the horn. Reconnect the cables to the battery, positive first, negative last. Press the horn switch on the steering wheel. The horn should work. If the horn doesn't work, or continues to blare, you will need to call on professional assistance to determine the cause.
Always keep a few basic tools and electrical tape in your car to deal with unexpected problems.