Have you checked that freeview is available in your area if not check with this link:
http://www.freeview.co.uk/availability
If you have done this and there is still no signal then double check through the set up menu and try another channel search.
If this still comes up with no signal, then it could be that your aeriel has not enough gain to pick up the signals. You would then have to replace yours with a suitable one. It could be that there is a poor connection to your aeriel lead this can often cause enough signal loss to prevent the freeview signal being recieved .
Are the analog signals being recieved O.K.?
Russ