Lawnmower Won't Start - Electrical?
Put it in neutral, and short the terminal on the solenoid, that the battery (+) is connected to which is a big lug, to one of the smaller terminals on the solenoid (the one that is not labelled as black in the drawing).  Assuming that they're not both black that should be easy.  There are three possibilities:  1.  There is a click and the starter motor starts to crank.  2.  There is a click and nothing else.  3.  No click, no nada.  Scenario #1 means that either your ignition switch or the clutch brake pedal up switch or the C-G contacts of the PTO switch are stuck open. It is still possible to start and use it safely if you have it in neutral and the other terminals of the start switch work (key must be set to run).  The one time that this happened to me it was the ignition switch, and I just started it by shorting out the two terminals for a month before finally getting a switch.  Scenario #2 means that either the starter motor is defective or the solenoid is defective.  It's not recommended to operate the tractor if this is the case but you can test which of these is the problem by shorting the two large lugs momentarily (just tap them lightly with something large).  If nothing happens it is the starter motor that is defective.  If it cranks then it is the solenoid that is defective.  Scenario #3 means that the solenoid is defective and needs to be replaced.  It won't be the fuse or the ammeter those are in parallel with the starter circuit.