You tried menu/select combined and it did not work? Does iTunes recognize it when you plug it in? you tried disk mode by doing this:
Before placing the iPod into Disk Mode, verify that it is charged.
Toggle the Hold switch on and off (set it to Hold, then turn it off again.)
Press and hold the Menu and Select buttons for at least six seconds, until the Apple logo appears.
When the Apple logo appears, release the Menu and Select buttons and immediately press and hold the Select and the Play/Pause buttons until the Disk Mode screen appears.
Connect the iPod to your computer; the screen on the iPod will should say "Do not disconnect". ? If any of this failed, you might need a new HDD or even logic board. Not sure about the Apple store, but if you are not comfortable to investigate further, it might be a good idea if you do have them check it. What iPod is it?