Minolta Dynax 40 and Minolta 3500xi Flash

I wonder and hope if anyone could help me. My parents have bought me this camera and flash for my birthday. Fantastic camera, over the moon with it. Only problem is, when I attach the external flash to the camera, the camera won't take a picture (like the button is jammed). The display just flashes up with the flash cancelled symbol. I've tried another flash and the same thing happens again. I know that it's not the flash gun, as my Dad has the same camera and he's had it working on his. Is there something stupid I haven't done, or is there something i need to alter on the settings on the camera??? Please Please help
Kind Regards