Hi. My switched off my Imac yesterday and when I came to switch it on today it wont start up. There's no sign of life at all. I have taken out anything that is connected and still have the same problem. If I pull out the power cord from the back of the mac and then plug it back in and press the power button, there is a noise like it is about to start up but then it just lapses back into silence and stays powered down. So it doesn't appear to be a power cord issue. Any help gratefully received. Thanks.