It sounds like your LNB is failing, or the cable has become loose.
To change between Horizontal, and Vertical, channels your receiver sends a signal, to the LNB, to switch between 13v, and 18v, Vertical, and Horizontal, polarity, when it stops switching it's usually due to the LNB, but might be other factors. If you can check your receiver, on another dish, a friends, or relatives, then you can rule out any problem with your receiver.