Most likely it’s a safety switch that is preventing the tractor from starting. Now assuming when you turn the key to the on position you should have power to your gauges, the fuel gauge should read tank level. If that’s the case I typically run through each safety device on the tractor (Neutral start switch, pto switch, seat switch). If everything is in neutral yet it still will not crank then I typically make a small jumper wire which helps determine which safety switch may be faulty by bypassing the switch and completing the circuit. If doing this solves the problem then you can go one step further and ohm test the switch to verify it is not completing the circuit. Then it’s just as simple as purchasers new safety switch.