If you are getting the message "no signal" this would indicate that there is a problem with the signal from your dish into your box. Even without a card inserted into a SkyDigibox, if there is a signal, you will always get Sky Guide and the free channels.
Try powering off your box, remove the card, check carefully the cables, especially the connection to the dish, reapply the power and then when asked to insert your card you should then be able to see (for example) channel/SNWS">Sky News on 501 and the BBC News channel on 503.
Once you can see any channel, reinsert your card. This should, after about one minute, give you the channels you expect, such as 103, 104 and 105.
If you continue to get "No signal", you dish may have been misaligned, or there may be a fault with the cables and connectors, or a problem with the Digibox. You can see the signal strength by pressing SERVICES, then 4-SYSTEM SETUP and 6-SIGNAL STRENGTH.
Judith Shaw: Should repeated switching between BBC1 & 2 fail to make any difference to the situation, then try disconnecting the box from the mains for about 30 seconds or so before reconnecting it again, as the problem might have been caused by a glitch in the system.
Although prior to carrying out this procedure, if your box is of the Sky+ variety? try swapping the LNB1 & 2 inputs over and see if this changes anything.
By the way, channel changing on a satellite box is achieved by the box sending instructions (voltage and tone) to the LNB mounted on the end of the dish arm, needless to say, these control signals can be shorted out if any water manages to creep into the LNB's "F" connector plug on the end of the coax.