You are scanning from the wrong area code.
Or the channels are down in your area at the moment.
But more likely the first answer.
From the installation menu, just restore to factory default settings, delete
all existing channels and rescan on another
area code.
You can repeat this until you get a
full channel list it won't do your box any harm.
If you don't delete previous searches first
box loops with "already exists" during the search and
the channel list won't change