Freeview isn't available through a Sky box. What you get is FreeSat from Sky, which is a different service (with a different selection of channels). Sometimes channels are removed from the line-up of a particular service, so the first thing to do is to check that the channels you require are still in the current listings of FreeSat from Sky:
http://www.freesatfromsky.co.uk/freesat-tv-cho ice.aspx
If the required channels are still in that list, your Sky box is obviously not receiving a strong enough signal on the relevant frequencies. Check that your satellite dish hasn't been knocked out of position (e.g. by high winds) and that it's not being obstructed (e.g. by a tree coming into leaf). If there's no obvious cause, you'll need to call in an aerial specialist to find out why your system has insufficient signal strength to resolve all channels.
Chris