I would first check the Seat Switch & the the Parking Brake Switch...
On the NH TC series the seat switch and the parking brake switch will not stop the engine from turning over however they cause the Fuel Solenoid to shut off. This will allow the engine to crank but not start.
You should also check to see if the Fuel Solenoid is activating when the key is turned run position. The fuel solenoid is on the right side of the tractor. If you set the parking brake and turn the Key to the ON position, you should hear it click.