Oh dear Stef, I hope I am wrong, but it sounds as if there is a problem with the power supply to your HD. Might it be your HD has died, how old is it? It could be just a loose wire in your power supply. It doesn`t sound like a software problem to me but i could be wrong.
I have to go out but no doubt someone else will come with a "Pearl of Wisdom".Might it be safer to remove the `puter plug from the power supply in case of a short circuit and fire, if you are leaving it on its own.