The shifting mechanism is quite simple and it could be that the shift linkage simply came apart. If you look underneath the tractor you should be able to see the rod that slides in an out of the transaxle to move the gear from high to low. The rod is on the left side above and forward of the axle and is about 5/8" diameter. If you move the shift lever and that rod does not move in and out then you have a problem with the linkage, if the rod does move then you'll have to take a peek inside the transaxle to figure out what's going on.