The tach on your tractor is most likely cable driven instead of mechanical.
The cable runs from the engine to the back of the guage.
Looks just like the speedometers and cables from older autos.
Odds are the cable has broken and no longer turns the meter.
If you can gain access to the rear of the guage and disconnect the cable,you could start the engine and see if the cable is turning.
If it isn't turning,you've found your problem.
Good Luck!
