There are four main reasons that the temperature gauge will not move. 1. the gauge is broken. Not a common occurrence and not the most likely unless there is visible damage to the gauge. 2. The temperature sensor on the engine block is broken. This is a common fault and a likely candidate to check for. 3. The thermostat is stuck in the open position. This is also a common fault and should be checked out, especially is the tractor has not been in daily use. 4. The wire lead from the temperature sensor to the temperature gauge is broken. This is a less common fault but should not be overlooked. This is actually the first thing to check because it is often the easiest thing to check for.
I hope this helps. Good luck and let us know what you find out.