I'm assuming you have the standard 16x16 PowerShuttle transmission - correct?
The common thing to look at is the seat switch. If it's gone bad, the transmission will be disabled and I don't know if a code throws on that one. I think the transmission disabled light should light up if you're trying to drive and the seat switch has disabled the transmission, but I'm not totally sure about that one... Easiest way to check the seat switch is to jump the connections on it so that it thinks there's always an operator in place. Not a good long-term solution, but it will help you narrow it down. If it is the seat switch that went bad, it's an easy fix.
If it's not the seat switch, it's really hard to say if no codes have shown up. It's entirely possible that it's throwing codes but they aren't blinking on the dash because the light stopped working (do you have a cab on the tractor?). If you have a dealer nearby, it would probably be worth the few hundred bucks to have them send someone out with a diagnostic tool and pull the codes. That will let you narrow the problem down and you can decide whether to fix yourself or have them do it.
The TN tractors seem to be rather bullet proof - except for the electronics... Mine has given me great service over the last ten years, but I've unfortunately had to become educated around all the things that can and do go wrong with the PowerShuttle transmission.