try pulling the card and reseating it again. If you can wipe the contacts with an isopropil wipe that will get off any dirt that may be present, these contacts don't like finger grease. Check you havn't bent any pins on the cable as well.
Ok, looks like we've sorted it - apols if I'm not using the correct terminology...
The PC package came with 2 similar looking cables (could it be VGA and SVGA?) so he got out the other one, but this made no difference at first.
He also had another look at the graphics card, and said there were 2 ports, one with less contacts than the other, so he put the card in that one, and it's now working, however the screen definition is visibly a lot lower than before.
This same thing didn't work in either port with the old cable, or with the new cable in the port with more contacts.
Does any of this make sense? Does it mean we're going to need a new graphics card eventually?
Low resolution could be just a matter of selecting the right one. Right click on the desktop, select Properties - Settings. There you can select the screen resolution and Colour (Color) quality. You shouldn't need to buy a new graphics card unless the present one is faulty or not good enough for what it's required to do. What is more likely to help you is to identify your present card and then download the manual for it.