It is unusual for a hard drive to suddenly stop, usually you get noises or beeps beforehand.
Have you checked the power is actually getting to the hard drive, i.e. different power cable or can you check that cable on something else; I presume it is a USB drive, have you tried another USB cable, as USB cables have been known to fail.
Are you running it via an adapter(i.e. 4 USB ports running off one USB socket, as they can (and do) fail.
Hope this helps