If it's like most other modern cars, it's behind the headlamp, under the washer fluid resevoir, in front of the passenger's side front wheel.
It requires removing the wheel and tyre, then (carefully) removing the fender liner, making sure not to bust all the plastic clips in the process. Then drop the washer resevoir out through the wheel well.
Then, and only then, will you even be able to SEE the horn. let alone do anything to it.
This is the process I had to go through on my Neon to put my air horns in. When I was done I really wondered whether it was worth it or not!!
I would go with Oblivious on this one though and check the fuse. There's really very little can actually go wrong with the horn. But it could have blown the fuse. If the fuse is fine, I'd check the horn relay in the fuse panel that's under the bonnet. If that's ok, I'd check the actual horn button for proper operation next. If that's still ok, THEN suspect the horn.....