Your ipod shmuffle is powered by a lithium ion (lithium polymer) battery. These are great batteries because they have a high amount of energy for their weight allowing Apple to keep the ipod small and light.
However, the lithium ion battery is very susceptible to damage and even fire and possible explosion if they are not kept within a very strict temperature and voltage range.
To ensure this, Apple has added a 'battery management system' chip to the battery that runs the battery charging and discharging. This chip controls the current, the maximum and minimum voltage of the battery, and the charging algorithm. Also, if the temperature of the battery is too high or low, it will turn off the battery.
This is great as it protects the battery (and you) from damage. But there is a down side, and the downside is that this battery management system or protection circuit board as it is sometimes called, uses a very small amount of battery power itself. If you haven't used your ipod for a long time, and the battery wasn't at a full charge, it is possible that the battery management system drained your battery. The problem is that there is a minimum safe voltage below which a lithium battery should not be charged. Your battery has probable reached this voltage and so the protection circuit has purposely shutoff the charging circuit so you can't charge the battery.
It is also possible that over the long period of time that the ipod sat idle, some kind of damage occurred to the battery management system and as a result charging does not occur. I have seen this happen a lot with laptop batteries.
Edit: when I think about it, I thought that Apple had within their battery management system a policy for long term storage that would essentially turn off the battery management system to avoid over draining the battery. It might be that you just need to plug it in for a period of time to 'wake up' the circuitry and then it will charge. Try for 20 minutes or less and don't leave it alone when you try - the Apple website also suggests that it may take up to 20 minutes after long term storage for an ipod to start charging again.
Do not fully charge or fully discharge your device’s battery — charge it to around 50%. If you store a device when its battery is fully discharged, the battery could fall into a deep discharge state, which renders it incapable of holding a charge. Conversely, if you store it fully charged for an extended period of time, the battery may lose some capacity, leading to shorter battery life.
Power down the device to avoid additional battery use.
Place your device in a cool, moisture-free environment that’s less than 90° F (32° C).
From the Apple support website:
"Depending on how long you store your device, it may be in a low-battery state when you remove it from long-term storage. After it’s removed from storage, it may require 20 minutes of charging with the original adapter before you can use it."
What I was describing above, if the ipod is in storage long enough, the battery could be in a too-low state and not be able to be charged safely. Good luck.