I have a JD 5210 2000 model with 418 hrs. Yesterday I was taking it to mow a neighbors and I just lost my c gear. No bad grinding sounds just locked up in the middle of the road. I was able to get it into B gear and get her home. I now only have B gear that works. You can get it into a and c but when you depress the clutch it feels like the break is stuck.
It is obviously locked in B range and wont shift out. Now you need to figure out what broke shift linkage, shift fork, etc.
Put everything in neutral and pull the shift cover off. You will have to take the seat off to get to it. You might find something broken there then it is an easy fix. If you have to go deeper, you can remove the creeper cover on the right side to see if there is anything wrong. More than likely, you will have to split it to fix it if it is broken down there.