It's most likely a fault with one of the cables running from the LNB to the dish. Easy enough figure out which one. Look in the Services - System Test menu. It'll tell you whether you're missing the signal from connection 1 or 2.
To check that it's the cable and not the LNB, swap the two connectors at the LNB, if you still get signal on the same cable, the LNB is fine, and it is the other cable.
Remove the plug from the problem cable, and check that the shielding wires (thin copper cables woven around the plastic insulator) do not touch the central core (thicker copper wire inthe centre ). Reassemble the plug carefully, ensuring good connection between the cable copper and the connector.
If both cables work while connected to a single LNB socket, then there's an issue with the LNB. It may have more than two connections (most supplied with 4 now), try the others.
Let us know how you get on with that, and we'll work from there.