My aftermarket horn quit working once and I spent quite a while trying to figure it out. Then I realized it was right after I'd installed my Hogtunes amp and had disconnected everything from the battery for a while. Somehow when reconnecting everything to the battery, I put both horn wires on B+. As soon as I grounded it properly, it magically started working again.
Hopefully your solution is this simple! If not, others will be along to walk you through some trouble shooting.