Why would a horn not work on a 93 Honda civic how do you fix this?

If all connections to the horn are fine and the fuse in okay, then you might want to check the horn relay or the actual switch at the steering wheel.

Answer

You will have to check the horn unit to determine if it is broken. This can be done by cutting apart the wiring harness, and connecting wires from your horn to your battery. If it doesn't make noise, your horn is shot. You'll have to take the bumper off the car to get at the horn, another testament of great engineering on the part of Honda :)