At work we get this with the machines which have IBM Hitachi Deathstars
As bender suggests, unplugging the hard drive and plugging it in a few times is often enough to start it up again. At least long enough to get it working so you can make back ups and order a new one.
once started up, checked event viewer
start-> Settings->Control panel -> Administrativer tools -> Event viewer
look through logs see if their are hard disk io warnings.