2004 Vauxhall Meriva My Vauxhall Meriva Horn Stopped Workin

I have a 2004 (54 plate) Vauxhall Meriva 16v 1.6 Life and the horn was working when I bought it a couple of months ago, but now it has stopped.
I've checked the fuse, it's fine, but someone mentioned something about a "horn switch" - where would I find that? And what else could cause the horn to suddenly not work?

Thanks!