That suggests that the battery has failed.
Furthermore, if the battery is noticeably warm (or hot) when you try to charge it, that indicates (to me) that the battery is in very poor condition and that you should not try charging it.
So, a very gentle warmth (i.e. barely noticeable), may be okay, but if it's any more than that, stop charging it. If it gets too hot to touch, definitely stop charging it and consider disposing of it as it may be unsafe. You can take an old iPod to any Apple store, where they can dispose of it safely. Otherwise, find out where to take (dispose) of an electrical item safely that is local to you (i.e. your local authority).
In the end, if it will not charge, then it is probably time to dispose of it.