Try to put it in Test mode: reset the iPod pushing Home/Select and Menu keys, then as soon as Apple logo appears push Home/Select and Previous keys. If you can't do it, your motherboard is probably toasted, AFAIK. If it works, perform all the tests just to check what's wrong: you'll probably find some faults in the I/O section, under the Hard Drive menu. It could be the HD, the drive connector on the motherboard or some defective ICs.
In the last few days I fiddled with some iPod Classic boards and one of them couldn't even enter the Test mode, so I guess it was cold dead... but I could be wrong. Any iPod experts/Geniuses around here?