It depends on how much the not being able to walk lasts. If its about 30 sec to a min it may be because you're not cooling down properly.Your body gets into a rythum while working out and if you go from walking quickly to just trotting off the treadmill this will cause a sensation of not being able to walk for a little bit.Try slowing down 0.5 mph at a time 1 min for each segment and see if it goes away.