2003 cherokee laredo horn blew out of no where wont stop

Its the horn button and/or a broken wire completing the circuit, and bypassing the switch. You will find both under the center cover in the steering wheel. If an air bag is also there, disconnect the battery and wait 10 minutes BEFORE removing the cover. Also, do not disconnect the battery with out first installing a "memory keeper" low voltage power source in the cigarette lighter socket.
If you don't the radio and the whole car will lock you out and do nothing. Thank the theft system for that!!

It could also be the relay for the horns. Usually in one of the under hood power distribution boxes. Swap the relay for another identical relay and see if the horn stops blowing by itself.