Just thought I'd share since this was the first time I ran into this and it took about 20 minutes (time to get more done during the shorter days).

Doing my normal backhoe and loader work when the tractor shuts off and cannot be restarted (you can hear clicking but engine won't turn). Remembering some threads on the seat switches I target them..nothing wrong..moved on to pedal pivotting...nothing wrong again...listened to starter clicking on and off..works fine. Then I took a look below the PTO lever and long behold, the switch is fully sticking out even though the lever is all the up (not engaged). pushed in the switch and turned the key...bam, engine rolled...turns out both screws which holds the switch in place was loose and the switch slid back...