This is indeed a weird one.

Take your machine around to a friend's and try it there. If it doesn't work then I would take it back to the shop and tell the guy you want to buy a cable that works. He probably has loads of spare ones rescued from dead machines and monitors (I certainly have )

The two things that I do find strange are the fact that you get the MoBo light, and you do not seem to have any problems with other domestic appliances.

I would first check that the voltage setting is correct.