Could be a bad connection on the horn buttons on the sterring wheel. They work just by having the movement of the horn button bring two metal pieces together. If either one is cruddy - it wont ground the horn wire. Also could be a disconnected wire on the side of the steering column, where there are several connectors.
Yes, you can replace the horn switch membrane by replacing it with one from a junkyard (even if the airbag has deployed, obviously much cheaper). Ford doesn't sell replacement horn switches unless you buy the airbag.
Search this forum for 'horn switch' and you'll find a detailed post with instructions.
The horn button membrane breaks down after about 4-5 years if you use the horn about once a day. I've had to replace mine twice in the last 6 years. Just get a replacement from a junkyard and be done with it.
on the escort i used to have, the horn switch was stuck (why i got it so cheap) i took it to a mechanic and had them install a button to the left of the steering column to bypass the horn switch inside and it passed inspection. they did it for 50 bucks vs. however much it wouldve cost to explode the airbag, replace the airbag, and fix the switch.
You don't have to explode and replace the airbag to replace the horn switch. It's serviceable separately, even if Ford frowns on the idea and won't sell you the switch separately. Search the forums for detailed instructions.