Tractor Repair: 1972 farmall tractor model#1026

Question
I have a friend who has this tractor and im just starting to look at it it was running fine then on a cold winter day it started backed up then wouldn't move he has changed the filters he says where do i start to attempt to fix this. Ive been an auto mechanic for years dabbled in tractors looking for some guidance

Answer
Jess, my diagnosis could be incorrect, but I am thinking that IF the tractor still runs OK, then there may well be a broken transmission mainshaft, OR a shattered clutch plate....I would suggest checking all linkages, and opening all inspection plates to look for anything irregular, and if nothing is obvious, pull the tractor with a chain, forward and backward with the shifter in neutral...Listen for any obvious bad noises...but you'll probably have to crack the tractor in half, and check the clutch first. Inspect the throwout bearing, and then start looking at the mainshaft if that is not the problem. Good luck!