The horn is trying to blow. It's stuck. There's a set screw on the back of it which can tune the horn and probably get it working again. I might be wrong with the cheap-fix first. If I'm not, you might spend money first that you don't need to spend. Did anything else stop working? Sometimes, the horn is on the same circuit as something else, like the heater or the radio. I hope you checked all the fuses. Good- I thought so. Make sure your brake lights work too.