I discovered my horn was not working yesterday, how long for I don't know.
I first checked the fuse in the panel at the right end of the dash which is covered when the door is opened but that was OK.
99% of the time when your horn stops working it is caused by oxidization or corrosion of the connections to the horn, that's what was wrong with mine.
To access the horns you take off the louvered plastic splash panel behind the bumper on the left side. This is secured by one 10mm bolt, one Phillips screw and one plastic retaining plug, remove these and the panel will fold down.
Shown in Photo 2.
You can just see the lower of the 2 horns, this horn takes the feed from your horn button and is relayed to the top horn which you can hardly see or reach.
White plug shown in the Photo 1
Release the white plug from the bottom horn after pulling out the spring clip, (don't lose it) then work the plug on and off the horn connections a few times to clean it testing to see if the horn works, keep moving the plug on and off till it does.
To reconnect the plug insert the spring clip on the plug and push it back on until you hear a click.
If you need to access the top horn the front bumper will have to be removed, full instructions with photos in the link below..............