I don't have really solid information, but I've just had a similar experience and resolved it. My gen 6 120 gb ipod has been running Rockbox for several years; I just hooked it up to my (Linux) computer to put some new music on the device. I umounted the drive ok, and when I unplugged the USB cable the iPod just went dead. Linux didn't recognize it, Win 7 didn't recognize it, iTunes didn't either, nothing lit up when I tried to turn it on. Tried every possible combination of button pushing and holding.
Finally I decided to just plug in the USB to my running Linux box and go out for some dinner. An hour later Linux recognized the drive and mounted it, and just for insurance I used RockboxUtility to update the system before unmounting and rebooting successfully.
btw I'm quite sure the battery wasn't dead. Windows and iTunes will recognize a dead-battery iPod. I have no idea what this was. Wait, yes I do, the music I had loaded just before disconnecting the device and having it die was not playable on the newly revived iPod. Got a message like "cannot load" when I clicked on those songs. That MIGHT have caused the problem.