If you've recently done work on your handlebars, triple tree, or switch boxes and your horn and/or starter buttons no longer work there's a good chance that the buttons are not completing their circuits as intended. Those two buttons rely on a ground connection that goes through the handlebars themselves, to the handlebar mounts, and typically need to be jumped past the rubber dampers.
In many cases the switch boxes do no make good contact with the handlebars because they are painted or plated in some non-conductive material. This can be fixed by grinding or sanding some paint away where the switch boxes meet the handlebars, and where the bars meet the clamps.
The studs protruding from the bottom of the mounting clamps have rubber dampers isolating them from the triple tree. There should be at least one jumper wire that provides the necessary electrical connection past the rubber dampers to complete the circuit. If this is missing or not connected neither of your buttons will work as intended.