Let me preface this by saying I am totally unfamiliar with that tractor.
That aside, at least one poster says you have mechanically actuated wet brakes, not hydraulic ones, which is in agreement with my experience with tractor brakes on older machines. I have never had one with hydraulic brakes.
Operating on the presumption that your system is mechanical and based on your description of the conditions under which they quit operating, I'm pretty sure you caught something in the linkage and have a part jammed or disconnected somewhere. Get under the tractor and have someone else move the brake pedal. Watch the linkage to see if everything moves all the way back to where the linkage disappears. Look for stretched, bent, or disconnected parts. If everything appears normal, consider a trip to the dealer to do the same thing on a new tractor and look for any differences.
The worst case I can think of is that a branch snagged a part of the linkage and bent or broke something internal. I think it is most likely that it broke something external -- there is usually an intentionally weak link in a complex mechanical linkage, like a shear pin on an outboard to protect the engine when the prop hits a rock.
Good luck and let us know what you find.
Like I said, this is based on zero familiarity with your machine, but experience with assorted mechanical systems.