I had something occur on my 06 400 and after resolving it I looked at the wiring diagram on the 650 and determined it could happen there also. It would not surprise me if the 07+ 400 is wired the same way. What happened was an aftermarket horn I was using shorted internally and that created a short through the mounting bolt. The way the horn is wired is that power goes to the fuse then to the horn then to the horn button and back to ground. That means that the horn always has power to it even if the horn button is not pressed. I was just riding down the road when mine shorted and I suddenly lost my dash gauges, blinkers and brake lights as these are all on the same circuit. The bike would also not restart after I stopped it because the brake lights didn't have power to them.
On the stock horn the mounting bracket has a rubber isolater built into it that keeps it from grounding if the horn shorts. The mounting bracket for the aftermarket horn did not have one. If you have mounted an aftermarket horn using the stock wiring and did not use the stock mounting bracket with it's isolater then you need to check to see if your mounting bracket has one. If not you need to add one.
If you ran a separate circuit through a relay to power the horn you would also not have a problem.
On the stock horn the mounting bracket has a rubber isolater built into it that keeps it from grounding if the horn shorts. The mounting bracket for the aftermarket horn did not have one. If you have mounted an aftermarket horn using the stock wiring and did not use the stock mounting bracket with it's isolater then you need to check to see if your mounting bracket has one. If not you need to add one.
If you ran a separate circuit through a relay to power the horn you would also not have a problem.