When a hard drive is failing to work properly (or not at all) try this neat trick. I believe it will only work on problems regarding the hard drive head mechanism and so if you are hearing strange noises (the correct terminology is head clatter) when the machine is switched on then this trick may help you.
Put the hard drive into a freezer. Yes you heard right, into a freezer (for about 15 minutes, although some people find better results after 12 hours), this can "unstick" the heads and temporarily get the hard drive working again. By freezing the hard drive, it causes the components to shrink which can enable the track marker to align with the tracks again (as a badly worn out or damaged hard drive may be misaligned and hence the strange mechanical noises it makes when booting up).
I'd like to add that this is not a proven method for fixing a hard drive, it tends to work on old hard drives a lot more than newer ones, and it is a last ditch solution to recover any badly needed data.
According to Pseudo_Intellectual this is referred as "the Novell trick", suggesting that it first came into practice there.