Just bought a new PC and monitor gets no signal - it works with the old PC
What is the most common reasons for this
Have tried the helpline but is closed
Have looked at manual and it states the most obvious reason for this is that the graphics card has moved in transit but am not sure what to look for when I open the case

First thing I would do is to check and see if there is more then one spot to connect the video cable to.

Sometimes manufacturers will add a new video card into the system, but the PC will still send the signal out to another source... usually a connect built onto the motherboard (or onboard video). So see if there is only 1 spot to plug the video cable into.

Here is a picture of a motherboard.
http://images.pcworld.com/howto/graphics/143531-motherboard.jpg

If your video cable plugs into a port that is in the upper right corner of this picture then you have an onboard video card and that can not be adjust or "re-seated" as your directions say. This area is also where you would plug in your mouse and keyboard.

If your video cable plugs into a card near then upper left (where the blue, white, and black plastic bars are) then you can try pulling it out and pushing it back in gently and seeing if that works.

Check those two situations first and see if that corrects anything.
And all in all.. ALWAYS CHECK YOUR CABLES.. sucks trying to figure something out just to realizes a cable has come loose.