I have a Yamaha YT6800 that the ignition switch mysteriously stopped turning. I moved it out of the garage, it sat for a couple of hours, then when I went to start it to put it back in the garage the ignition would not turn.
I have no machine specific suggestions, but any time I have a key switch that won't turn the first thing I try is giving a light shot of graphite powder or spray into the lock, just in case it's a stuck tumbler or a bit of corrosion jamming things up. Lock de-icer, if you live in a cold climate, does the same. Never hurts to try the simple fix first.