If it is an newer tractor, 2008 and after, the tractor has a "power on" fuel system. It is easy to check the fuel systems on these tractors. When you turn the key to "on", you will hear a pretty loud clap from the solenoid on the injector pump. You can hear it from the seat. If you don't hear the clap, you have a power problem in the tractor and it definitely won't start. The fuel rack is still closed.
If it is a tractor from before 2008, it has a "power off" fuel system with the kill knob below the ignition switch. We have a seen a few tractors where the shut-off solenoid sticks after shut-off holding the rack closed.
In either case, it is a pretty easy fix. First determine what type of shut-off system you have. Trouble shoot from there.
Let us know if we can help.