Unplug the box from the mains and leave it unplugged for at least 5 mins, if not longer.
Then plug it back into the mains, and again leave it, this time for about 3-5 mins.
Then switch it on and press the backup button to clear the message.
Now try BBC or ITV.
If you still have problems, check the signal quality under 'Services'. '4 System Setup' and then '6 Signal Test'.
If the signal strength is low and keeps moving up and down, it may be that your dish has moved.