I would start by reprogramming both remotes with this procedure from the Techinfo section.
On the horn button not working. Have you confirmed that the horns are indeed good or that power is not reaching the connectors on them?
If no, please use a multimeter set to DC volts while someone presses the horn, ground the black meter lead and touch the horn contact with the red lead. 12V should be read. This test would indicate that the horn is bad.
If power is present. The airbag/horn was probably put back on incorrectly at one time. This requires disconnecting power from the and following the airbag safetly procedures. Then pulling it an looking for something obstructing the pressing of the horn.