Your horn button energizes a solenoid that opens to allow air to flow to the horns. Issue is either with the horn button or the solenoid. You can test horn button and solenoid with a test light and a jumper wire.
Your dash lights (and maybe the horn too) could be out because of a bad ground. Check under the dashboard for a loose ground wire. Again a jumper wire could be used to make a temporary ground to check.