Horn

Always follow the most obvious when faced with an electrical accessory that has stopped working: FUSE. Look in your owner manual for your fuse location. Then stop by an autozone or a similar store and if they are not too busy they will "usually" go outside and help you replace a fuse if you can't do it yourself. ( is easy though, I bet you can do it. :) ) If its not the fuse then things can get a little more complicated, such as a relay, the horn itself, etc.

Possible bad clock spring.