1968 a833 1-2 gear stuck

Did you try and turn the output shaft and move the 1-2 lever? If not pull the side cover and see what is wrong. If yours is a 1968 year trans it will have the ball and detent system to keep from shifting into two gears at once. I would check to see if that was assembled wrong. If that appears correct have a look at the slider ring to see if it can be moved.