I got this a while ago, after some research I found that with quite a few people the hard drive head had simple got "stuck".
I banged it against a table on the side of the iPod rather than the back or front then pressed the central button to boot it up, and it worked!
It's happened a few times since but each time i just wack it against my other hand and all is fixed.
The best way of diagnosing this fault it to hold it up to your ear, if it sounds like the hard drive is trying to spin up then failing quickly by clicking spinning down and stopping, making this sound repeatedly and frequently then it's probably this issue.
This saved me from replacing the hard drive in it, my fault it broke anyway as i dropped the iPod onto a tiled floor.