Everything eletrical shut off

Could be ignition fuse. If not that then check the large block connectors which are usually located on the left hand side at the front of the bike behind the cowling beneath the handle bar. These connectors are notorious on GSXR's

you need to replace the air temperature sensor. looks like this one https://www.ebay.co.uk/itm/Suzuki-GSXR-TL-SV-AN-600-750-1000-Air-Temperature-Sensor-1996-2009-13650-61B00-/182447458716?hash=item2a7ab7499c:g:-rYAAOSwopRYmblq

Well...in general....the turn signals brake lights and horn and instrument lights are typically on the Signal fuse on most bikes.
So....any recently added accessories to the circuit?
If not...start at the signal and brake light sockets...inspect wiring around the socket and the sockets themselves looking for a possible short.
Then check wiring to the back under seat for chafing pinching etc
Inspect the wiring aroiund the steering head in case a pinch has occured causing a short on bar rotation.
If all looks okay...inspect the horns....not sure on your Busa, but many horns use 2 wires and ground through the horn button....so the horn needs to be insulated on its mount so as it may be wired hot and can short.

what is your idle set at? try turning it up a little. should be around 1100 - 1300 rpms. or you might need a valve adjusment.

It has injectors.Does it display any "C" codes when you crank it up? Did you ever tinker any Fuel Injection modules or sensors? Have you inspected/ or visual checked or try to replaced your fuel filter?

have you changed the flasher relay to run led indicators if not you will need to you cant run standard bulb indicators and led together. and with the brake like sounds like pulling on the cable when on lock causing switch to stay on check that location of the wiring is correct and sufficient freeplay

your speedo gear is loose or broke check both it is located in front brake housing

please check the battery terminals, the wire might have been displaced from the terminals or have a loose connection

check corroded wires at connector of the left hand switch coupler