Hello, Had a message appear on my iMac "you need to restart your computer hold down the power button for several seconds or press reset". Upon doing so the machine will or does not seem to be able to boot into the OS. When I press the power button to turn it on it gets stuck on a black screen, there is no startup chime, just a whirring sound from inside the machine (not sure if this is the HD or optical drive). Any suggestions as to what to do? I have tried the PRAM reset and am not able to boot into safe mode, from a disc or get to the command prompt (or whatever its called on a mac). Many thanks for any advice offered as I am stumped.