have you checked the fuse?
also,whats known to go is the brass ring that sits behind the steering wheel and thats what touches the brass nipple.
when you push the horn,the brass ring touches the brass nipple and there is contact.
The brass ring(or nipple) wears out and then there is no contact,so have a look at that.
You can put some gasket maker behind the ring to make it stick out more and therefor touch the nipple.