I had this same problem yesterday right after I jailbroke my wife's iPod Touch (4th Gen). If your device is jailbroken, I would suggest trying to boot it tethered. Once you do that, you can unplug it and you shouldn't have any problems with it not booting up. This is how I eventually fixed my wife's, and it now boots up just fine.
You can boot your iPod in tethered mode in Redsnow.
Redsnow Download Link
In order to do this, you'll need to load up the corresponding IPSW for your current iOS. This can be found by doing a quick google (e.g. IPSW 4.2.1 download). Just replace the 4.2.1 with whatever iOS you are using.
Hope that helps!