Disconnect the "Kill" wire where it junctions on the side of the "Throttle Box" on the side of the engine and see what happens then. While anything can fail, B&S coils are very reliable. I suspect the "Interlock Module" under the plate on the right side of your mower. OR Perhaps the finger that the shifter lever depresses operating the neutral switch has become bent, not depressing the switch.

Walt Conner