Try putting it in the freezer for a bit, it may work.
Try taking the Hard Drive out and use another enclosure, maybe the enclosure is broken
Note: If/when it works, DO NOT TURN IT OFF, copy as much of your data off right then and there, it may be the last time it'll spin up again.
This may help: http://www.wikihow.com/images/0/0f/FlowC...
I know I'm resurrecting a dead thread but for anyone searching it may come in handy.
Usually the only time the freezer trick works is when the drive is making a constant clicking, grinding/squeal
Freezing the hard drive causes the bearing and other moving parts to shrink slightly, possibly freeing them up. They will spin freely until the drive heats up, so make sure you get your most important files off first.
If it only clicks when trying a specific operation or when opening a certain file/folder/app
As for enclosures being faulty, I have had that happen many times with cheap enclosures. The bridge inside goes and the drives can make clicking noises and sound like they are going to die. Switch them over to a new enclosure, and they're good to go.