I have a John Deere L120 lawn mower. The mower deck just cut off with no warning, sound or vibration. What has gone wrong?

Hi,
Far and away the most common issue is broken wires. This usually occurs on the right side inside the frame near the clutch and can most often be fixed with crimp connectors. Another is broken or shorted wires in the rear harness. The usually occurs behind the rear rear wheel/tire along that side on the frame. Still another is the breking of the clutch stay which causes it to spin and break the wiring. The stay attaches to the left frame beside the engine by two bolts. There are two varieties that work the same. Then there is clutch failure.
One thing to note is that a working clutch and its switch can be easliy tested by sitting on the seat, turning the key switch to Run and pulling out on the PTO knob to engage (engine not running) and you will hear the clutch engage.
Peace,
Clarke