What Murph said. Look down on the left side for a lever on the fuel pump. That lever is pulled by a solenoid and will have a red tip on it, though it is tough to see if you are in a hurry to shut it down. Look down behind the dash for it.
Causes could vary, but could be a blown fuse. If the tractor is already off, turn the key on and you'll hear the fuel pump click like crazy. If you turn the key off, the pump stops, but then the solenoid should kick in and it lasts for about 15-20 seconds. Really made me curious when I first got mine. I'd walk away and then here this loud click about 5 steps away. That was the solenoid releasing.