I bet it's the shifter fork, they have a tendency to snap. First and reverse share a shifter fork. I had the same thing happen. It's a simple repair. The top of the transmission comes off with a few bolts. Clean it well before taking it off so you don't get crud in your gearbox. The fork is held on by a drift pin, I think about a 16d nail is the size to drive it off. Drive it off, slide the broken fork off, slide the new one on and drive the pin back on.
If you take the top off you'll quickly see if the fork is the problem. You don't have to worry that taking the top off will introduce leaks, the seal is well above the oil line.
Don't drive the tractor until you get it fixed, if a piece falls into the transmission it will cause you a world of hurt.